ACP: Add `bytes` to the standard library

(As discussed with @Darksonn.)

Proposal

Problem statement

Support storing and modifying buffers of bytes, including efficient references to subsets of those buffers without copying, and disjoint modifiable buffers. Provide a common vocabulary type to be used pervasively throughout the ecosystem, as a replacement for Arc<[u8]>.

Motivating examples or use cases

This is commonly needed when working with buffers of data and passing them around throughout a program, especially async programs (for the same reason that async programs very commonly make use of Arc). Bytes provides for Arc<[u8]> what &[u8] provides for Vec<u8>, and BytesMut additionally allows copy-on-write behavior.

Solution sketch

I'm proposing to add more-or-less a full copy of the bytes crate to alloc::bytes, including Bytes, BytesMut, Buf, and BufMut. Before stabilizing, we can do a review with the bytes maintainer, @Darksonn, and make sure we don't propagate any legacy APIs that have been on the bytes todo list to clean up, if any.

What happens now?

This issue contains an API change proposal (or ACP) and is part of the libs-api team feature lifecycle. Once this issue is filed, the libs-api team will review open proposals as capability becomes available. Current response times do not have a clear estimate, but may be up to several months.

Possible responses

The libs team may respond in various different ways. First, the team will consider the problem (this doesn't require any concrete solution or alternatives to have been proposed):

• We think this problem seems worth solving, and the standard library might be the right place to solve it.
• We think that this probably doesn't belong in the standard library.

Second, if there's a concrete solution:

• We think this specific solution looks roughly right, approved, you or someone else should implement this. (Further review will still happen on the subsequent implementation PR.)
• We're not sure this is the right solution, and the alternatives or other materials don't give us enough information to be sure about that. Here are some questions we have that aren't answered, or rough ideas about alternatives we'd want to see discussed.

Start by reading the ACP proposal and its feature-lifecycle guidance, then compare the proposed alloc::bytes APIs with the existing bytes crate. Done means the libs-api team has reviewed whether this belongs in the standard library and clarified the API scope before implementation.