Goals

Functorise solver.ml with term module and theory module
- SAT solver depends on actual boolean literals
- Move terms and theories (SMT part) to another directory
Add proof output as resolution
- Each theory brings its own proof output (tautologies), somehow
- pure resolution proofs between boolean clauses and theory tautologies
Allow to plug one's code into boolean propagation
- react upon propagation (possibly by propagating more, or side-effect)
- more advanced/specific propagation (2-clauses)?