Added AVL tree implementation to moorings, based tree-set on top of it
Adding oracle-style tests for sets
Improved set tests
Merge branch 'tree-sets' of git://github.com/ejcaruso/pyret-lang into ejcaruso-tree-sets
trying to pull out sets into another library, hitting cyclic problems

This commit may get reverted
mostly working, but broken by a separate issue of generative constructors
trying to use placeholders for list/set instantiation
Merge branch 'master' of github.com:brownplt/pyret-lang into ejcaruso-tree-sets
Move sets into a separate library

Giving up on:

- .to-set() shared method on lists
- list.to-set() on the list object

These required mutual dependencies between moorings and other libraries, which
are tricky and we don't have a great solution for.

sets are still available in the core language, but to convert to a list you
need to do more explicit calls into the set library
merge
update tests