Satisfiability Assertions

[sidprasad]

Adding support for assertions about the satisfiability (or unsatisfiability) for assertions. These take the form:

assert <predicatename> is sat
assert <predicatename> is unsat
assert <predicatename> is forge_error

[tnelson]

@sidprasad Looks good. I made some minor revisions; if they look OK to you, go ahead and merge in.

I believe the answer to the comment I deleted is "yes", assuming it meant that both options were valid (not that they might be valid together).