fix #6343

The bug was that axiom generation was not enabled on last_index, so no axioms got created to constrain last-index.
With default settings the solver is now very slow on this example. It is related to that the smallest size of a satisfying assignment is above 24. Pending a good heuristic to find initial seeds and increments for iterative deepening, I am adding another parameter smt.seq.min_unfolding that when set to 30 helps for this example.

src/ast/rewriter/seq_axioms.cpp

@@ -509,6 +509,8 @@ namespace seq {
     }
 
     /**
+       i = last_indexof(t, s):
+
        !contains(t, s) => i = -1   
        |t| = 0 => |s| = 0 or i = -1
        |t| = 0 & |s| = 0 => i = 0