WIP: refactor to generalize Machine::run_top_level()

src/bin/scryer-prolog.rs

@@ -6,6 +6,6 @@ fn main() {
         scryer_prolog::machine::INTERRUPT.store(true, Ordering::Relaxed);
     }).unwrap();
 
-    let mut wam = machine::Machine::new();
-    wam.run_top_level();
+    let mut wam = machine::Machine::new(Default::default());
+    wam.run_top_level(atom!("$toplevel"), (atom!("$repl"), 1));
 }