Investigate Cost of Making TrieMap and TreeMap's iterators DoubleEnded

[Gankra]

This will definitely pump up their implementation complexity, and probably their memory footprint a bit. Depends on the precise implementation. Still worth implementing to see. The proposed range() iterators would naturally fall out of such an implementation, though.

[Gankra]

cc @aturon

[Gankra]

TrieMap is not in std lib anymore.