Fixed bug with mutable closures in interpret (not really --- up to closure recursion)

regression/test068.expr

@@ -0,0 +1,19 @@
+fun lazy (f) {
+  local flag = 0, value = 0;
+
+  return fun () {
+    if flag
+    then return value
+    else
+      value := f ();
+      flag  := 1;
+      return value
+    fi
+  }
+}
+
+local l = lazy (fun () {write (1); return 800});
+local x = read ();
+
+write (l ());
+write (l ())