Some fixes and tests, begin the implementation of type inequalities solving.
Move Mvar into the core.
Cleanup, subsequent to the integration of Mvars into the core.
Intermediate commit: new unification; solves inequality constraints.
Inequalities resolution works, modulo a correctness bug.

Now one can no longer specify a special tactic for solving the TCCs but rather simply which constants to unfold. We may go back to the more general case if needed.
Corrected soundness bug in unification.