Add support for ref.null in global initializers (#1131)
As per the reference-types spec, ref.null is a constant expression.
I did try to add support for ref.func at the same time, but I didn't
really understand the strategy of wabt and reference types; there seems
to be strong intertwingliness between func.ref and the element section,
whereas it does seem possible for a func.ref to be global. Oh well, I
didn't need it :)
