Begin work on a left-leaning red black tree

This commit isn't entirely ready but my hard drive is about to die and
I want to back up what I have so far...
Replace first draft of llrbtree with a directly translation of the Sedgwick paper

Includes basic tests for insertion and search.
Fix typo in basic BST test that caused it to not test anything
white space
white space
Rename test to reflect that its role will expand as features are added
Make LeftLeaningRedBlackTree Pythonic by implementing collections.abc interfaces
Further make LLRBTree Pythonic by making some class methods regular methods
More Pythonic-ization: Make _Node._insert a regular, not class, method
More Pythonic: Move search to Node class
More Pythonic: make insert part of Node's public interface
typo in method call
typo in method call
Do not allow BST keys that are not orderable
Be more cautious about non-orderable keys
Test that BSTs support non-hashable keys
Test BSTs with random keys
Unnest the Node class from the LLRBTree class
Fix up docstrings
Use dummy _Node class instead of lots of `if _root is None` conditions