New C++ Api: First batch of API guards. (#4557)

New C++ Api: First batch of API guards. (#4557)

This is the first batch of API guards, mainly extending existing guards
in the Solver object with checks that Ops, Terms, Sorts, and datatype objects
are associated to this solver object.

This further changes how DatatypeConstructorDecl objects are created. Previously,
they were not created via the Solver object (while DatatypeDecl was). Now, they are
created via Solver::mkDatatypeConstructorDecl, consistent with how DatatypeDecl
objects are created.