zz: remove command argument type-checking wrapper.

* lib/zz/eval.lua (marshaller): Remove.
(Defun): Remove "marshall-argtypes" symbol property. Set __call
metamethod to call function directly rather than through the
marshaller wrapper.
(Defun): Remove argtypes parameter.  Adjust all callers.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>