Rewrite get_many_mut methods

[JustForFun88]

The previous form of the HashMap::get_many_mut_inner and HashMap::get_many_unchecked_mut_inner functions was not very optimal:

• firstly, the old RawTable API was not very convenient and did not combine well with each other (get_mut requires that eq: impl FnMut(&T) -> bool, get_many_mut requires that eq argument should be a closure such that eq(i, k) returns true if k is equal to the ith key to be looked up.).
• secondly, the hash of all keys was calculated, regardless of whether there are any of given keys in the table at all.

[Amanieu]

I'm not actually sure this is an improvement, have you benchmarked this? It adds a lot of branches in the generated code to handle lazy hash computation. Straight up computing the hashes and doing multiple lookups might result is faster code. In practice, this API is used when you have few keys (usually just 2) and you're already pretty sure all are in the table.

[bors]

☔ The latest upstream changes (presumably #528) made this pull request unmergeable. Please resolve the merge conflicts.