handle non-list cases in '', add '' as a built-in (re: issue #31)

src/prolog/macros.rs

@@ -763,6 +763,12 @@ macro_rules! cyclic_term_execute {
     )
 }
 
+macro_rules! skip_max_list_execute {
+    () => (
+        Line::Control(ControlInstruction::CallClause(ClauseType::SkipMaxList, 4, 0, true))
+    )
+}
+
 macro_rules! return_from_clause {
     ($lco:expr, $machine_st:expr) => {{
         if $lco {