- sidekick.model removed, now just smtlib.Model (specific to it)
- use function entries for models, not just term->term
- re-building models in smtlib driver
- asolver.solve, in Check_res.t, does not return a concrete model, but a
bundle of functions to query the solver
- store constants in smtlib typechecker AST (so we can directly map them
to values in model construction)
