std::hashmap: Implement external iterator for HashMap and HashSet
std::hashmap: Add test_iterate for HashSet
std::hashmap: Use .iter() instead of .each and similar
std::hashmap: Remove BaseIter impl for HashSet

Remove the BaseIter impl, while keeping the .each method until callers
are converted.
std::to_str: Use .iter() for HashMap and HashSet
auto merge of #7319 : thestinger/rust/hashmap-iterator, r=thestinger

#7277