`prove` and `sat` fails on proof scripts of types other than `ProofScript SatResult`

[brianhuffman]

The types of prove and sat indicate that they should work with any argument of type ProofScript b, but they fail in many other cases:

sawscript> prove (return ()) {{ True }}
saw: fromValue SatResult: ()

sawscript> prove assume_valid {{ True }}
WARNING: assuming goal prove is valid
saw: fromValue SatResult: Valid

sawscript> sat (return ()) {{ True }}
saw: fromValue SatResult: ()

sawscript> sat assume_valid {{ True }}
WARNING: assuming goal sat is valid
saw: fromValue SatResult: Valid