Add a make_tuple() function to loopy.

This function does trivial things, but it's there to solve the problem
that the reduction neutral element getters are not allowed to store
dtypes (#80).

The function mangler demands that a function knows its type based on
its arguments. For the neutral element getters, this is impossible
because they take zero arguments. The simplest fix I can think of is
to change a call to neutral_element() to a call to make_tuple().

Currently, the tuple code doesn't work yet due to pickling issues.  I
think the root cause is somewhere in
__hackily_ensure_multi_argument_functions_are_scoped_private().