Hex string representation of small digest values should have leading zeros #214

Whenever we represent a digest as a hex string, we should encode it in a big-endian way to make small¹ digest values have leading zeros.

The behavior of the functions to_hex, try_from_hex, and the Ord implementation should be consistent. The conversion to and from BigUint also needs to agree.

Note that we do not necessarily have to change the encoding of other values. I am only concerned with the hex string representation of Digest. e.g. BFieldElement or Digest is serialized is not of a concern, I think.