Add more safe methods to RawTable

[cuviper]

These methods combine lookup and bucket operations so the caller doesn't
have to deal with unsafe bucket methods.

• get: find and as_ref
• get_mut: find and as_mut
• insert_entry: insert and as_mut
• remove_entry: find and remove
• erase_entry: find and erase

[Amanieu]

What is the rationale for adding these methods instead of just using the safe API in HashMap directly?

I have been considering going in the other direction: exposing more functionality in HashMap so that we can eventually deprecate RawTable.

[cuviper]

I was motivated for this by the cached crate's SizedCache, which was using HashMap<K, usize> and LRUList<(K, V)>, where the latter is indexable by the usize values from the map. I thought it would be nice to remove the K duplication by using RawTable<usize> instead of the map, reading the keys from the list -- jaemk/cached#59. However, dealing with Bucket would introduce the first unsafe in that crate, avoided by this PR's safe methods.

That proposed use case is quite a bit like indexmap, where key storage is managed independently. Another example I've been playing with is this autohash crate, where keys can report their own "hash" without actually going through a Hasher, as a better solution than rust-lang/rust#76233 when the key is already hash-like. So I think RawTable has really nice flexibility, and I'm skeptical that the same uses could be served by HashMap extensions.

[cuviper]

If anything, I might like to see RawTable pulled out into its own crate, for cases where the map/set implementations are not used. That could be forked out of hashbrown, but it would be nice to continue sharing that implementation.

[cuviper]

@Amanieu -- any more thoughts on this? We can close if you think it's not viable, but it seems simple enough to me.

[Amanieu]

Sorry, this slipped my mind. I think these are probably fine to add as is.

[cuviper]

And I see you also published 0.9.1, thanks!