Add support for passing axioms to provers

[atomb]

Along the lines of similar work in SAW (GaloisInc/saw-script#1037), and building on the same infrastructure, it may be useful to allow Cryptol users to pass universally-quantified axioms to solvers. Given that we already have a notion of properties, which are implicitly universally quantified, this could look something like:

:proveWith lemma1, lemma2 : goal

where lemma1 and lemma2 are the names of already-specified properties, and goal is a Cryptol expression of the same form as can be currently passed to :prove.