Auto merge of #51569 - SimonSapin:liballoc, r=sfackler

Auto merge of #51569 - SimonSapin:liballoc, r=sfackler

Make the public API of the alloc crate a subset of std

This only affects **unstable** APIs.

I plan to submit an RFC proposing to stabilize the crate. The reason it isn’t stable yet (https://github.com/rust-lang/rust/issues/27783) is in case we end up merging the standard library crates into one. However the `core` crate is already stable, so if that happens we’ll need to keep it working somehow (likely by making replacing its contents by `pub use` items). We can do the same for `alloc`. This PR will hopefully make this easier, but even if that doesn’t happen consistency with `std` seems good.