Remove the result_X() methods; move that logic into the Digest impls.
Create cryptoutil.rs and re-write the Sha2 module to make use of it.
Re-write the Sha2 compression functions to improve performance.
Add little-endian version os read_u32v and write_u32.
An imlementation of MD5.
Remove DigestUtil and convert its methods to default methods on the Digest trait.