Basic structure for the integration of Z3 into Congress.
Defines a Z3Theory as an alternative to nonrecursive theories.
Defines a Z3Context for executing Z3Theory content.
Z3 must be installed manually and is not in the requirements
devstack will install Z3 if ENABLE_CONGRESS_Z3 is set.
Limitations:
* No built-ins
* Typechecking does not support sub-typing (will be done later through built-ins).
Partially-implements: blueprint alternative-engine-z3
Change-Id: I87ff439a3ed4a3e83c78c98add7d94275f716a01
c5f633dfa2