Subclass BaseAuthTreeLink instead of using level_compress flag. This now has the logic for link hashing inside of the AuthTreeLink classes instead of being scattered about.
Alter structure of the hash tree such that node metadata is encoded in leaf nodes only, greatly simplifying the structure of the hash tree itself.
Switch from sha256 to hash256 (double-SHA256), eliminating early optimization in favor of cryptographic belts-and-suspenders and standardization with other bitcoin primitives.
Update unit tests to pass with latest changes to bitcoin.authtree.