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.

source/toy_ast.h

@@ -81,6 +81,8 @@ typedef enum Toy_AstFlag {
 	TOY_AST_FLAG_POSTFIX_INCREMENT = 43,
 	TOY_AST_FLAG_POSTFIX_DECREMENT = 44,
 
+	TOY_AST_FLAG_INVOKATION = 45,
+
 	// TOY_AST_FLAG_TERNARY,
 } Toy_AstFlag;