Access to ScriptPubKeys of inputs.

[roconnor-blockstream]

It is suggested in https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2018-November/016508.html that signatures cover the input's scriptPubKey:

 * The hash of the scriptPubKey (or nothing if MASK)

Simplicity's Bitcoin primitives should be expanded to make this easy as well.