Replace std::hashmap::{each_key, each_value} with iterators

[Dretch]

It feels surprising to me that each_key and each_value still use the old closure-based iteration protocol.

I think it would be better if they were replaced with keys and values methods that return new-style iterators.

I can send a pull request, unless there are objections.

[gereeter]

This is fixed with the merging of #11242 and should be closed.