hashmap: Remove .mutate_values() which is replaced by .mut_iter()
hashmap: Iterators for hashset diff, sym. diff, intersec, union

Implement the difference, union, etc iterators with the help of a custom
iterator combinator with explicit closure environment. Reported issue #7814
to be able to use the std::iterator filter combinator.
auto merge of #7815 : blake2-ppc/rust/hashmap-iterators, r=huonw

Implement set difference, sym. difference, intersection and union using Iterators.

The set methods are left since they are part of the Set trait. A grep over the tree indicates that the four hashset operations have no users at all.

Also remove HashMap::mutate_values since it is unused, replaced by .mut_iter(), and not part of a trait.