Add subcommand to run program without generating transaction from it

[schoen]

Another developer request from the hackathon: it would be great to have a command to test-run a specified Simplicity program (in a specific transaction environment, so presumably still from a PSET, since we need to be able to reference in the inputs and outputs!), "just to see what happens" without specifically generating an updated transaction. This would probably be something like the update-pset command without actually updating the PSET, but ideally also generating some amount of debugging output, in the future including additional debugging outputs that isn't available yet.

For example, this would combine really well the idea of mocking jet returns (#23), because one could see whether a program would succeed or fail given a specified behavior of a jet (e.g., does this program succeed if this specific signature verification succeeds, even though the developer doesn't actually possess a witness that makes it succeed?).