Functions are working, tests incomplete

This required a massive cross-cutting rework to the scope system,
multiple subtle bugfixes and relearning of the parser internals, but it
does appear that functions are working correctly.

A few caveats: for now, parameters are always constant, regardless of
type, return values can't be specified, and some script tests have been
written.

Most importantly, a key feature is working: closures.

scripts/leapyear.toy

@@ -1,8 +1,9 @@
-//BUG: Not yet functional
-
 //find the leap years
 fn isLeapYear(n: int) {
 	if (n % 400 == 0) return true;
 	if (n % 100 == 0) return false;
 	return n % 4 == 0;
 }
+print isLeapYear(1999);
+print isLeapYear(2000);
+print isLeapYear(2004);