solver: check and set type to reconcile class and proto

solver: check and set type to reconcile class and proto

the solver checks its proto type (SolverParameter.type) on
instantiation:

- if the proto type is unspecified it's set according to the class type
  `Solver::type()`
- if the proto type and class type conflict, the solver dies loudly

this helps avoid accidental instantiation of a different solver type
than intended when the solver def and class differ. guaranteed type
information in the SolverParameter will simplify multi-solver
coordination too.