Add binary representation of hash-size (#399)

Add binary representation of hash-size (#399)

* Add binary representation of hash-size

Fixed size efficient binary representation data,
instead of the variable size hexadecimal string.

This is suitable for machine use, while the other
is more suitable for human readability as string.

* Promote hex format as a global function

* Add unittest for the format_hex function

Make sure that output is NUL-terminated