I think initializing structs should (almost) be always done with designated
initializers, because otherwise it's easy to get the order wrong. The
problem is that otherwise the order of fields gets additional meaning
not only for the memory layout, but also for the code that initialize
the structs.

Add a macro NM_VARIANT_ATTRIBUTE_SPEC_DEFINE() that replaces the other
(duplicate) macros. This macro also gets it right to mark the struct as
const.