Rename and expose BytesMut::spare_capacity_mut

[yotamofek]

This PR renames BytesMut::uninit_slice to spare_capacity_mut, to match Vec::spare_capacity_mut, and makes it a public API.

There are two reasons I'm proposing this change:

• Even though BufMut::chunk_mut has a similar purpose, the fact that BufMut supports non-contiguous buffers makes unsafe code using it on a BytesMut type somewhat harder to reason about. The chunk_mut docs explicitly mention the returned slice might be less than buf.capacity(), which is untrue for BytesMut (because it is always contiguous), but not very clear when reading code using it.
• <BytesMut as BufMut>::chunk_mut has additional logic to check whether the buffer is currently at full capacity, and reserve 64 additional bytes if it is. This is a redundant check when calling buf.reserve(n) followed by buf.chunk_mut(), but unfortunately the optimization process doesn't seem to be able to eliminate the unreachable branch. I've seen this to have, admittedly small, negative impact on performance in certain scenarios.

IMHO, spare_capacity_mut is a more descriptive name and I thought it would be helpful to match the std naming, which is why I renamed the method.
I've also aped the docs from the std docs for Vec, but slightly reworded them to fit BytesMut :)

[Darksonn]

I am okay with adding this now that Vec::spare_capacity_mut is stabilized in the standard library, but I think the return type should be &mut [MaybeUninit<u8>] because it is not UB to de-initialize the data in the region.

[yotamofek]

Thanks for the review, fixed!

[Darksonn]

Looks good. Just one doc comment.

[Darksonn]

Thanks.