Support for the raw_entry API

[thomcc]

This isn't yet part of std::collections::HashMap's API, but has a number of features that cannot be done in the current API. It would be extremely nice if these could be done with hashbrown.

For background, see:

• Raw entry Pre-RFC: https://internals.rust-lang.org/t/pre-rfc-abandonning-morals-in-the-name-of-performance-the-raw-entry-api/7043
• And the rust-lang PR: Add raw_entry API to HashMap rust#54043

[Amanieu]

map.rs in hashbrown is pretty much a straight copy & paste of map.rs from the standard library, so porting raw_entry should be trivial.

[Amanieu]

There is a slight issue, see my comment.