Wrap hex.DecodeString() into mustDecodeHexString()

Wrap hex.DecodeString() into mustDecodeHexString()

The hex decoding is only done from the JSON files provided by the
wycheproof-testvectors package. Failure is always fatal. So there
is no need for repeated error checks, and we can use an ergonomic
wrapper.

Also rework the calculation of the message digest from input data
this had a similar deficit.

All in all this shaves off about 10% of the code and removes a lot
of tedious repetition.