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.

tests/units/test_function.c

@@ -0,0 +1,9 @@
+#include "toy_console_colors.h"
+
+#include <stdio.h>
+#include <string.h>
+
+int main(void) {
+	printf(TOY_CC_WARN "Test not yet implemented: %s\n" TOY_CC_RESET, __FILE__);
+	return -1;
+}