no_std support

[MaciejWas]

Hi, thanks for writing and sharing this crate!

I wanted to use it in a no_std environment but I noticed that there is no std/no_std feature flag. I modified your crate in my fork and got quadtree to compile without the std library (7fef19c). Would you accept a PR adding no_std support?

I looks like the only std-incompatible thing is the usage of std::collections::HashMap, so my proposition would be to replace it with hashbrown + ahash (which appears to be a better fit than the default hasher). Other option would be to allow users to specify the hasher.

[JohnDowson]

A long while back I've tried making this crate generic over storage. Since then impl-trait-in-traits been stabilized, making that old draft PR feasible. A HashMap is not the only and not necessarily the most optimal type for backing storage.