Add two public API functions on the two public keys, both called
`p2wpkh_script_code` to do exactly as the name suggests.
Of note, I was not able to find anywhere to use these in example code,
this is because of we always use the new `p2wpkh_signature_hash`
function. The new functions may be useful for a user calling
`segwit_v0_encode_signing_data_to`. The may help document the library as
well.
3c62f74684