Switch hashbrown to std::collections::hash_map

[andersk]

The std::collections::hash_map implementation was replaced with a copy of hashbrown in Rust 1.36. There’s still a difference in the default hasher, but we disabled that default in #205. So I think this switch has no effect but to improve compile times and binary sizes.

[andersk]

Oh, looking further through the history, I see there was a change in #194 to use the raw_entry API that’s not yet stabilized in std, although it was then reverted. Are there plans to bring that back?

[notgull]

I'd prefer hashbrown here, if not only for the prospect of future no_std support