* 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.
