Add methods for obtaining iterators over the keys and values of a TreeMap
Add methods for obtaining iterators over the keys and values of a SmallIntMap
Add methods for obtaining iterators over the keys and values of a TrieMap
auto merge of #15968 : nham/rust/keys_values_other_maps, r=alexcrichton

Adds methods for obtaining iterators over the keys or values of a SmallIntMap/TreeMap/TrieMap.

Closes #14376