test(hansbug): add benchmark for the basic operations and functions

dev(hansbug): refactor all the __ixx__ operators

dev(hansbug): optimize for TreeValue's getattr

dev(hansbug): add support for debugger in TreeValue class

dev(hansbug): add get_or_default method to test_storage

feature(hansbug): add flatten and unflatten to support parallel calculation.

fix(hansbug): fix bug when missing is used on the whole non-existing child tree

feature(hansbug): Add __version__ to treevalue module