define more types / logic in CplexSolver.fs

[smoothdeveloper]

sketching few ideas for shape of a backend:

• the internal solver backend has a module called Primitives with simplest functions for doing simple operations that helps setup the bijection between Flips and backend for variables and constraints state
• there will be logic that prepare, run and extract results of the Flips model, and it should be similar among 2 given backends

we will see how we make the whole thing polymorphic from Flips.Solver project standpoint, can be several solutions.