The function tHash shifted the hash number to an incorrect number of bits
when the bit string had leading zeros. This is one of two issues reported
in vincenthz/hs-tls#152.
use the typeclass for the lowest IO impure C bindings definitions,
and define the pure interface as generic on top of this.
At the same time define an Hash.IO interface to allow mutable manipulations
of hash contextes when necessary.
Use HashAlgorithm instead of HashFunction in the [PubKey] sections
Tweak the HMAC, PBKDF2 functions to be more efficient and use the new interface
