Implemented iterator for TrieMap

Closes #5506.
Remove debug printing.
auto merge of #8293 : dim-an/rust/trie-iterator, r=thestinger

Closes #5506.
Updated std::Option, std::Either and std::Result

- Made naming schemes consistent between Option, Result and Either
- Changed Options Add implementation to work like the maybe monad (return None if any of the inputs is None)
- Removed duplicate Option::get and renamed all related functions to use the term `unwrap` instead