This method computes the weight an InputWeightPrediction
would to a transaction, not including witness flag bytes.

The P2WPKH_MAX constant assumed DER signatures in the witness have
a max length of 73. However, their maximum length in practice is 72,
because BIP62 forbids nodes from relaying transactions whose ECDSA
signatures are not canonical (i.e. all sigs must have an s value of
less than n/2). This means s is never encoded with a leading zero
byte, and the signature as a whole never exceeds 72 bytes in total
encoded length. The ground_p2wpkh function was already correct;
only the constant needed to be corrected.

Sanity checks the InputWeightPrediction against
a transaction which uses P2WPKH inputs.