hashed-storage 0.4.0

• Index now uses a 32-byte representation for sha256 hashes, instead of a 64-byte ascii-hex (base16) encoding.
• New module Storage.Hashed.Hash that exports a new incarnation of Hash data type and a number of utilities.
• The Tree type is overloaded over a monad type and is not bound to IO monad anymore.
• Index has been streamlined, the API has improved safety and the code has been extensively optimised.
• Support for creating new hashed trees with the - format used in darcs 2.0.2 and newer has been dropped.