Split mutable methods out of Set and Map

Fixes most of #4989. I didn't add Persistent{Set,Map} since the only
persistent data structure is fun_treemap and its functionality is
currently too limited to build a trait out of.
Un-broke benchmarks
auto merge of #7768 : sfackler/rust/containers, r=huonw

See #4989. I didn't add Persistent{Set,Map} since the only
persistent data structure is fun_treemap and its functionality is
currently too limited to build a trait out of.