* Refactor Hash code to simplify md5 and sha256 implementations
Md5 and Sha256 (and other future digests) require less boilerplate code
now. Error reporting includues the name of the hash again.
* Add missing hash sha256 test
Hashers now uses on Rust Crypto Digest trait which makes it trivial to
implement additional hash functions.
The original `md5` crate does not implement the Digest trait and was
replaced by `md-5` crate which does. Sha256 uses already included `sha2`
crate.
