This lets us take Bytes and a &[u8] slice that is contained in it, and
create a new Bytes that corresponds to that subset slice.

Closes #198

With this if foo is a mutable slice, it is possible to do

foo.into_buf().put_u32_le(42);

Before this patch into_buf would create a Cursor<&'a [u8]> and it
would not be possible to write into it.

- Clones when the kind is INLINE or STATIC are sped up by over double.
- Clones when the kind is ARC are spec up by about 1/3.

The intent of the license was to dual license MIT & Apache 2.0. However,
the messaging was copy / pasted from rust-lang.

Clarify the license as exclusively MIT.

Fixes #215

* Recycle space when reserving from Vec-backed Bytes

BytesMut::reserve, when called on a BytesMut instance which is backed by
a non-shared Vec<u8>, would previously just delegate to Vec::reserve
regardless of the current location in the buffer. If the Bytes is
actually the trailing component of a larger Vec, then the unused space
won't be recycled. In applications which continually move the pointer
forward to consume data as it comes in, this can cause the underlying
buffer to get extremely large.

This commit checks whether there's extra space at the start of the
backing Vec in this case, and reuses the unused space if possible
instead of allocating.

* Avoid excessive copying when reusing Vec space

Only reuse space in a Vec-backed Bytes when doing so would gain back
more than half of the current capacity. This avoids excessive copy
operations when a large buffer is almost (but not completely) full.

The new implementation tries to get the data directly from bytes() (this is
possible most of the time) and if there is not enough data in bytes() use the
previous code: copy the needed bytes in a temporary buffer before returning
the data

Here the bench results:
                               Before                After           x-faster
get_f32::cursor             64 ns/iter (+/- 0)    20 ns/iter (+/- 0)    3.2
get_f32::tbuf_1             77 ns/iter (+/- 1)    34 ns/iter (+/- 0)    2.3
get_f32::tbuf_1_costly      87 ns/iter (+/- 0)    62 ns/iter (+/- 0)    1.4
get_f32::tbuf_2            151 ns/iter (+/- 18)  160 ns/iter (+/- 1)    0.9
get_f32::tbuf_2_costly     180 ns/iter (+/- 2)   187 ns/iter (+/- 2)    1.0

get_f64::cursor             67 ns/iter (+/- 0)    21 ns/iter (+/- 0)    3.2
get_f64::tbuf_1             80 ns/iter (+/- 0)    35 ns/iter (+/- 0)    2.3
get_f64::tbuf_1_costly      82 ns/iter (+/- 3)    60 ns/iter (+/- 0)    1.4
get_f64::tbuf_2            154 ns/iter (+/- 1)   164 ns/iter (+/- 0)    0.9
get_f64::tbuf_2_costly     170 ns/iter (+/- 2)   187 ns/iter (+/- 1)    0.9

get_u16::cursor             66 ns/iter (+/- 0)    20 ns/iter (+/- 0)    3.3
get_u16::tbuf_1             77 ns/iter (+/- 0)    35 ns/iter (+/- 0)    2.2
get_u16::tbuf_1_costly      85 ns/iter (+/- 2)    62 ns/iter (+/- 0)    1.4
get_u16::tbuf_2            147 ns/iter (+/- 0)   154 ns/iter (+/- 0)    1.0
get_u16::tbuf_2_costly     160 ns/iter (+/- 1)   177 ns/iter (+/- 0)    0.9

get_u32::cursor             64 ns/iter (+/- 0)    20 ns/iter (+/- 0)    3.2
get_u32::tbuf_1             77 ns/iter (+/- 0)    35 ns/iter (+/- 0)    2.2
get_u32::tbuf_1_costly      91 ns/iter (+/- 2)    63 ns/iter (+/- 0)    1.4
get_u32::tbuf_2            151 ns/iter (+/- 40)  157 ns/iter (+/- 0)    1.0
get_u32::tbuf_2_costly     162 ns/iter (+/- 0)   180 ns/iter (+/- 0)    0.9

get_u64::cursor             67 ns/iter (+/- 0)    20 ns/iter (+/- 0)    3.4
get_u64::tbuf_1             78 ns/iter (+/- 0)    35 ns/iter (+/- 1)    2.2
get_u64::tbuf_1_costly      87 ns/iter (+/- 1)    59 ns/iter (+/- 1)    1.5
get_u64::tbuf_2            154 ns/iter (+/- 0)   160 ns/iter (+/- 0)    1.0
get_u64::tbuf_2_costly     168 ns/iter (+/- 0)   184 ns/iter (+/- 0)    0.9

get_u8::cursor              64 ns/iter (+/- 0)    19 ns/iter (+/- 0)    3.4
get_u8::tbuf_1              77 ns/iter (+/- 0)    35 ns/iter (+/- 0)    2.2
get_u8::tbuf_1_costly       68 ns/iter (+/- 0)    51 ns/iter (+/- 0)    1.3
get_u8::tbuf_2              85 ns/iter (+/- 0)    43 ns/iter (+/- 0)    2.0
get_u8::tbuf_2_costly       75 ns/iter (+/- 0)    61 ns/iter (+/- 0)    1.2
get_u8::option              77 ns/iter (+/- 0)    59 ns/iter (+/- 0)    1.3

Improvement on the basic std::Cursor implementation are clearly visible.

Other implementations are specific to the bench tests and just map a static
slice. Different variant are:
 - tbuf_1: only one call of 'bytes()' is needed.
 - tbuf_2: two calls of 'bytes()' is needed to read more than one byte.
 - _costly version are implemented with #[inline(never)] on 'bytes()',
   'remaining()' and 'advance()'.

The cases that are slower (slightly) correspond to implementations that are not
really realistic: more than one byte is never possible in one time

- All the `get_*` and `put_*` methods that take `T: ByteOrder` have
  a `where Self: Sized` bound added, so that they are only usable from
  sized types. It was impossible to make `Buf` or `BufMut` into trait
  objects before, so this change doesn't break anyone.
- Add `get_n_be`/`get_n_le`/`put_n_be`/`put_n_le` methods that can be
  used on trait objects.
- Deprecate the export of `ByteOrder` and methods generic on it.

Fixes #163 

* Handle empty self and other for unsplit.
* Change extend() to extend_from_slice().

If `shallow_clone` is called with `&mut self`, and `Bytes` contains
`Vec`, then expensive CAS can be avoided, because no other thread
have references to this `Bytes` object.

Bench `split_off_and_drop` difference:

Before the diff:

```
test split_off_and_drop             ... bench:      91,858 ns/iter (+/- 17,401)
```

With the diff:

```
test split_off_and_drop             ... bench:      81,162 ns/iter (+/- 17,603)
```

Add support for unsplit() to BytesMut which combines splitted contiguous memory blocks efficiently.

Fixes #164

* Compact Bytes original capacity representation

In order to avoid unnecessary allocations, a `Bytes` structure remembers
the capacity with which it was first created. When a reserve operation
is issued, this original capacity value is used to as a baseline for
reallocating new storage.

Previously, this original capacity value was stored in its raw form. In
other words, the original capacity `usize` was stored as is. In order to
reclaim some `Bytes` internal storage space for additional features,
this original capacity value is compressed from requiring 16 bits to 3.

To do this, instead of storing the exact original capacity. The original
capacity is rounded down to the nearest power of two. If the original
capacity is less than 1024, then it is rounded down to zero. This
roughly means that the original capacity is now stored as a table:

0 => 0
1 => 1k
2 => 2k
3 => 4k
4 => 8k
5 => 16k
6 => 32k
7 => 64k

For the purposes that the original capacity feature was introduced, this
is sufficient granularity.

* Provide `advance` on Bytes and BytesMut

This is the `advance` function that would be part of a `Buf`
implementation. However, `Bytes` and `BytesMut` cannot impl `Buf` until
the next breaking release.

The implementation uses the additional storage made available by the
previous commit to store the number of bytes that the view was advanced.
The `ptr` pointer will point to the start of the window, avoiding any
pointer arithmetic when dereferencing the `Bytes` handle.

* Inner: make uninitialized construction explicit
* Remove Inner2
* Remove unnecessary transmutes
* Use AtomicPtr::get_mut where possible
* Some minor tweaks

Slice operation should return inline when possible

It is cheaper than atomic increment/decrement.

Before this patch:

```
test slice_avg_le_inline_from_arc   ... bench:      28,582 ns/iter (+/- 3,880)
test slice_empty                    ... bench:       8,797 ns/iter (+/- 1,325)
test slice_large_le_inline_from_arc ... bench:      27,684 ns/iter (+/- 5,920)
test slice_short_from_arc           ... bench:      27,439 ns/iter (+/- 5,783)
```

After this patch:

```
test slice_avg_le_inline_from_arc   ... bench:      18,872 ns/iter (+/- 2,937)
test slice_empty                    ... bench:       9,136 ns/iter (+/- 1,908)
test slice_large_le_inline_from_arc ... bench:      18,052 ns/iter (+/- 2,981)
test slice_short_from_arc           ... bench:      18,200 ns/iter (+/- 2,534)
```