Use Open/Close Scope without Local (i.e., export the scope opening)
only when the scope corresponds to the main purpose of the module.

Due to Coq bug #10480 or #10474 it actually used `Morphisms.solve_proper`
instead of the version of std++. The version in std++ can inherently
not solve this, so I changed it into a manual proof.

This avoids `naive_solver` tearing whole goals apart, even if the goal
appears exactly as a hypothesis.

This closes issue #36.