Cover entire HashMap API for the HalfBrown map

[Licenser]

The HalfBrown map covers the essentials of HashMap but not everything quite a few things are missing.

[Licenser]

Moving this to the halfbrown repo - it is about 90% done already mostly missing entry and related functions.

[Licenser]

@shekohex you seemed interested in the issue is there a particular function you need that is still missing?

[shekohex]

Everything is fine, Thank you for that great crate ❤️

[wbrickner]

I am trying to use halfbrown as a drop-in replacement for std::collections::HashMap.

one method I am sorely missing is HashMap::retain<F>(&mut self, f: F).

[Licenser]

@wbrickner added and released as 0.1.12 👍