Add of_hex and of_bin functions

[MisterDA]

It's easy to serialize a ShaX.t value: just convert it to its
hexadecimal or binary representation. However, the library doesn't
provide any means of unserializing the value.

This commit introduces functions to unserialize ShaX.t values from a
binary or an hexadecimal representation of a digest.

I'd like to share the hex_to_int and of_hex functions between the
modules, would you be fine with adding an util.h file to the
library?

[djs55]

Looks useful! Feel free to add a util.h.

[MisterDA]

Done! also switched to ounit2 and added myself to the list of authors.