First-stab at AVL tree set.
Reviewer comments
More cleanup.
Add .from() -> Iterator<V>
BugFix: Make bidirectional iterators work.

TreeSet was returning Iterator<V> for iterator and reversed, which didn't
break the contract with Iterable<V>; however it should have been returning
bidirectional iterators, which was clearly not working.

Added tests for noraml iterators as well as new from() iterator.
Moving tests to "treeset_test.dart"

Just clean up, no code change.
Reviewer Comments

reversed -> reverseIterator
from -> fromIterator
Comment clarifications
Reduce externally visible api.
Perf: improve union, intersection, and difference if both are TreeSets.
Overhaul AvlTreeIterator and Tests

-Reviewer comments
-Large overhaul of AvlTreeIterator to support 'from' as well as base
iteration.
-Re-worked tests for better readability
Privatize _AvlTreeIterator
More comments / cleanup
Merge pull request #94 from jtmcdole/treeset

TreeSet collection