Native functions now only get the Interpreter& as an argument. They can
then extract |this| along with any indexed arguments it wants from it.
This forces functions that want |this| to actually deal with calling
interpreter.this_value().to_object(), and dealing with the possibility
of a non-object |this|.
This is still not great but let's keep massaging it forward.
Now that Interpreter keeps all arguments in the CallFrame stack, we can
just pass a const-reference to the CallFrame's argument vector to each
function handler (instead of copying it.)
