* use slice.to_vec instead of buf.put in From<[u8]>

* don't panic in fmt::Write for BytesMut

Return empty `Bytes` object

Bench for `slice_empty` difference is

```
55 ns/iter (+/- 1) # before this patch
17 ns/iter (+/- 5) # with this patch
```

Bench for `slice_not_empty` is

```
25,058 ns/iter (+/- 1,099) # before this patch
25,072 ns/iter (+/- 1,593) # with this patch
```

Round up to power of 2 is not necessary, because `reserve` already
doubles previous capacity in

```
	new_cap = cmp::max(
		cmp::max(v.capacity() << 1, new_cap),
		original_capacity);
```

which makes `reserve` calls constant in average. Avoiding rounding
up prevents `reserve` from wasting space when caller knows exactly
what space they need.

Patch adds three tests which would fail before this test. The most
important is this:

```
#[test]
fn reserve_in_arc_unique_does_not_overallocate() {
    let mut bytes = BytesMut::with_capacity(1000);
    bytes.take();

    // now bytes is Arc and refcount == 1

    assert_eq!(1000, bytes.capacity());
    bytes.reserve(2001);
    assert_eq!(2001, bytes.capacity());
}
```

It asserts that when user requests more than double of current
capacity, exactly the requested amount of memory is allocated and
is not wasted to next power of two.

`extend_with_slice` is super-convenient operation on `Bytes`.

While `put_u8` would be expensive on `Bytes`, `extend_from_slice`
is OK, because it is batch, and it checks for kind only once.

Patch also adds `impl Extend for Bytes`.

cc #116

Similar to `Vec::extend_from_slice`: it a reserve followed by
memcopy.

The shared debug_assert is to ensure that the internal Bytes
representation is such that offset views are supported. The only
representation that does not support offset views is vec.

Fixes #97

Before this commit `Bytes::split_{off,to}` always created a shallow copy if `self` is arc or vec.

However, in certain cases `split_off` or `split_to` is called with `len` or `0` parameter. E. g. if you are reading a frame from buffered stream, it is likely that buffer contains exactly the frame size bytes, so `split_to` will be called with `len` param.

Although, `split_off` and `split_to` functions are `O(1)`, shallow copy have downsides:

* shallow copy on vector does malloc and atomic cmpxchg
* after shallow copy, following operations (e. g. `drop`) on both `bytes` objects require atomics
* memory will be probably released to the system later
* `try_mut` will fail
* [into_vec](https://github.com/carllerche/bytes/issues/86) will copy

Standard `Debug` implementation for `[u8]` is comma separated list
of numbers. Since large amount of byte strings are in fact ASCII
strings or contain a lot of ASCII strings (e. g. HTTP), it is
convenient to print strings as ASCII when possible.

Limit the number of threads when using qemu to 1. Also, don't bother
running the stress test as this will trigger qemu bugs. Finally, also
make the stress test actually stress test.

Closes #79

I found this significantly improved a
[benchmark](https://gist.github.com/danburkert/34a7d6680d97bc86dca7f396eb8d0abf)
which calls `bytes_mut`, writes 1 byte, and advances the pointer with
`advance_mut` in a pretty tight loop. In particular, it seems to be the
inline annotation on `bytes_mut` which had the most effect. I also took
the opportunity to simplify the bounds checking in advance_mut.

before:

```
test encode_varint_small  ... bench:         540 ns/iter (+/- 85) = 1481 MB/s
```

after:

```
test encode_varint_small  ... bench:         422 ns/iter (+/- 24) = 1895 MB/s
```

As you can see, the variance is also significantly improved.

Interestingly, I tried to change the last statement in `bytes_mut` from

```
&mut slice::from_raw_parts_mut(ptr, cap)[len..]
```

to

```
slice::from_raw_parts_mut(ptr.offset(len as isize), cap - len)
```

but, this caused a very measurable perf regression (almost completely
negating the gains from marking bytes_mut inline).

Also fixes an issue with a line wrap in the middle of an inline code
block.

Closes #83

The `Source` trait was essentially covering the same case as `IntoBuf`,
so remove it.

While technically a breaking change, this should not have any impact due
to:

1) There are no reverse dependencies that currently depend on `bytes`
2) Source was not supposed to be implemented externally
3) IntoBuf provides the same implementations as `Source`

Given these points, the change should be safe to apply.

This change tracks the original capacity requested when `BytesMut` is
first created. This capacity is used when a `reserve` needs to allocate
due to the current view being too small. The newly allocated buffer will
be sized the same as the original allocation.