Working on RawEntry for stabilization in the std

[sanbox-irl]

Hello!

Stabilization of the raw entry api in the std is stalled here: rust-lang/rust#56167 (comment)

Copy/pasting what @Amanieu wrote in the linked comment here, for what needs to be done:

• We need proper documentation for the API functions, including examples.
• hashbrown has a few extensions to RawEntry, such as https://docs.rs/hashbrown/0.9.1/hashbrown/hash_map/enum.RawEntryMut.html#method.and_replace_entry_with. We should decide whether to port these to std as well.
• The name feels a bit wierd since it isn't clear what "raw" means. But I don't have any better ideas.

This all seems pretty accomplishable! I've never contributed to this crate (or Rust actually), but it seems mostly like just finding some examples and writing some docs.

I'd love to take a stab at it, but thought I'd make this issue here for a place to discuss if there's anything else that needs to be solved, especially with point #2 above about the various extensions to raw entry.