CNF conversion refactoring (#5547)

* split sat2goal out of goal2sat

These two classes need different things out of the sat::solver class,
and separating them makes it easier to fiddle with their dependencies
independently.

I also fiddled with some headers to make it possible to include
sat_solver_core.h instead of sat_solver.h.

* limit solver_core methods to those needed by goal2sat

And switch sat2goal and sat_tactic over to relying on the derived
sat::solver class instead. There were no other uses of solver_core.

I'm hoping this makes it feasible to reuse goal2sat's CNF conversion
from places like the tseitin-cnf tactic, so they can be unified into a
single implementation.

src/api/api_solver.cpp

@@ -42,6 +42,7 @@ Revision History:
 #include "sat/dimacs.h"
 #include "sat/sat_solver.h"
 #include "sat/tactic/goal2sat.h"
+#include "sat/tactic/sat2goal.h"
 
 
 extern "C" {