Fixed invoking functions within function arguments

Somewhat similar to c9d4b9965c

scripts/for-theory.toy

@@ -0,0 +1,31 @@
+//WARN: This is just a scratch pad, don't use it
+
+//for (var i in array) print i;
+//for (var i in range(10)) print i;
+
+
+//example of a `range` function
+fn range(limit: Int) {
+	var counter: Int = 0;
+
+	fn next() {
+		if (counter >= limit) {
+			return null;
+		}
+		else return counter++;
+	}
+
+	return next;
+}
+
+var next = range(10);
+
+
+fn log(x) {
+	if (x == null) return;
+	print x;
+}
+
+while (true) {
+	log(next());
+}