Make hashbrown dependency optional

[cccs-ki]

I'd like to use the Box part of this library, however I don't want to bring in any dependencies. I think there should be a feature called "hashmap" which gets added to the default features, but is required to use hashbrown and the hashmap functionality.

Thus, removing the feature should remove hashbrown as a dependency.

[baumanj]

Hashbrown is already the default for rust stdlib, so unless you’re using nostd, it’s not adding any new dependencies

[cccs-ki]

While the stdlib does use hashbrown, the dependency will still be downloaded and compiled, thus being one more package that needs to be vendored.