Builtins: the meta::invoke method on "Function" should not be final

[lth]

spec/library/Function.html: In the RI, the "meta::invoke" method for the Function object is final. It is clear that meta::invoke on direct instances of Function must be entirely magic, but it's not clear that this should prevent subclasses from overriding it: they can be treated like other objects that have "invoke" methods (and which already work fine in the RI).

If we consider meta::invoke on Function as taking no parameters then methods in subclasses can have various parameter lists without restriction, I believe.

I propose that we remove the restriction that this method be final.