Add flowtable interpreter (flowterp)

This pull request includes an implementation of a flowtable interpreter. This interpreter is used in quickchecking to ensure two things:

1. that the semantics and the LocalCompiler flowtables agree;
2. that the LocalCompiler respects the axioms of NetKAT when generating flowtables for a policy

Depends on frenetic-lang/ocaml-openflow#120.