Implement pluggable smtlib2 backends

[pschanely]

It would be great to let the user pick their own smtlib backend, but right now we are pretty heavily dependant on the z3 Python API. The Z3 python API is also quite slow, so I suspect that resorting to raw SMTLIB commands is unlikely to slow us down.

NOTE: Looks like we could try out the incremental parser and see how that does.