Tweak numbering of STP input files for ease of use

Make each file name from a run unique without needing to account
for the directory name as well.
Modify the STP library interface for the latest STP

Don't simplify the expressions for variable names before using them
to look up a counterexample, and if get_term_from_counterexample
returns a term that is neither a BVCONST (completely constrained)
nor a SYMBOL (completely unconstrained), fall back to get_counterexample
which should always return an BVCONST.