Replace uses of `fail` in Verifier.SAW.Simulator with `panic`.
Replace uses of `fail` in Verifier.SAW.Simulator with `panic`.

In all cases, `fail` was being called in situations that indicated
that a programmer error had occurred.

GaloisInc/saw-script,GaloisInc/saw-script,GaloisInc/saw-script,GaloisInc/saw-script,GaloisInc/saw-script