[std] haxe.ds.HashMap improvements

`haxe.ds.HashMap` has a number of shortcomings that make it more awkard to use than a regular `haxe.ds.Map`. None of them seem very hard to fix:

- [x] there are no array access operators, even though it's an abstract
- [ ] it does not unify with `Iterable`, meaning static extensions like `Lambda.count()` don't work on it - according to @RealyUniqueName this is because it's an abstract and could be fixed with an appropriate `@:to`
- [x] there's no key-value-iteration support (related: #7395)

There might be more I haven't run into.