<Arc<T>>::make_mut returns a &mut T, such an API is doable for Bytes too
and thus we should reserve Bytes::make_mut for that.
Furthermore, it would be helpful to use From<Bytes> as a trait bound
in some cases with other traits such as Hyper's body trait, where Hyper
gives you Bytes values.
Finally, making it impl From<Bytes> for BytesMut means the API is more
easily discoverable as it appears on both Bytes and BytesMut.
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
