new core perf - add merge_tf and enable_cgc distinction

perf fix for propagation behavior for equalities in the new core.
The old behavior was not to allow congruence closure on equalities.
The new behavior is to just not allow merging tf with equalities unless they appear somewhere in a foreign context (not under a Boolean operator)

The change re-surfaces merge_tf and enable_cgc distinction from the old core.
They can both be turned on or off.

merge_enabled renamed to cgc_enabled

The change is highly likely to introduce regressions in the new core.

Change propagation of literals from congruence:
- track antecedent enode. There are four ways to propagate
literals from the egraph.
- the literal is an equality and the two arguments are congruent
- the antecedent is merged with node n and the antecedent has a Boolean variable assignment.
- the antecedent is true or false, they are merged.
- the merge_tf flag is toggled to true but the node n has not been merged with true/false

src/sat/smt/arith_internalize.cpp

@@ -372,7 +372,7 @@ namespace arith {
         enode* n = ctx.get_enode(atom);
         theory_var w = mk_var(n);
         ctx.attach_th_var(n, this, w);
-        ctx.get_egraph().set_merge_enabled(n, false);
+        ctx.get_egraph().set_cgc_enabled(n, false);
         if (is_int(v) && !r.is_int()) 
             r = (k == lp_api::upper_t) ? floor(r) : ceil(r);        
         api_bound* b = mk_var_bound(lit, v, k, r);