deallocate old stack frames (#201), start using tags, fix a panic! associated with partial strings

src/prolog/machine/and_stack.rs

@@ -66,6 +66,11 @@ impl AndStack {
             }
         }
     }
+
+    #[inline]
+    pub fn truncate(&mut self, len: usize) {
+        self.0.truncate(len);
+    }
 }
 
 impl Index<usize> for AndStack {