Fix tests

Use case:

```
let bytes: Bytes = ...
let subbytes = bytes.slice(a..b); // where a == b
let slice = &subbytes[..];
let slice_bytes = bytes.slice_ref(slice);
```

Last line should not panic, because `slice` object is derived from
the original `Bytes` object.

Before this commit it panics, because `Bytes::slice` returns a fresh
`Bytes` object when `begin == end`.

* Remove incorrect comment about behavior of put_slice
* Improve grammar and clarity of BytesMut documentation

"Promotable" `Bytes` object is constructed from disassembling a
boxed slice object, not a vec.

Thus we should reassemble data into a boxed slice, not into a vec.

Although, it does not create any problems in practice (`Box<[u8]>`
is allocated exactly the same way as `Vec<u8>`), technically it is
a violation of `Vec::from_raw_parts` spec which says that a pointer
"needs to have been previously allocated via `String`/`Vec<T>`".

This separates the `SharedVtable` into 3:

- `PromotableEvenVtable`: The original `SharedVtable`, which will
  promote the `Vec` to `Shared` on the first clone, and is selected when
  the `Vec`'s pointer has the LSB unset.
- `PromotableOddVtable`: Similar to the `PromotableEvenVtable`, but
  selected when the `Vec`'s pointer has the LSB set. This vtable differs
  in the masking used when reconstructing the `Vec`.
- `SharedVtable`: This no longer checks if its current kind is `VEC` or
  `ARC`, and is only created by the "promotable" vtables.

This also adds a test using an "odd" global allocator that purposefully
bumps all pointers with alignment of 1.

Closes #343

Closes #340

When the length to truncate is greater than the buffer's current
length, do nothing instead of clearing the contents.

This type is the counterpart to BufExt::take. It limits how many bytes
some `BufMut` can actually be filled with.

remove either feature

This brings `BytesMut` in line with `Vec<u8>` behavior.

This also fixes an existing bug in BytesMut::bytes_mut that exposes
invalid slices. The bug was recently introduced and was only on master
and never released to `crates.io`.

In order to fix a test, `BufMutExt::chain_mut` is provided. Withou this,
it is not possible to chain two `&mut [u8]`.

Closes #170

Rust versions bumped to the minimum needed for const slice len.

Makes the short-circuit checks inline-able, and the moves the actual
reserving code to an inner function.