add API for setting variable activity

Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com>

src/smt/smt_kernel.h

@@ -229,6 +229,11 @@ namespace smt {
         */
         expr_ref_vector get_trail();
 
+        /**
+           \brief set activity of literal
+        */
+        void set_activity(expr* lit, double activity);
+
         /**
            \brief (For debubbing purposes) Prints the state of the kernel
         */