gives a more specific signature to `computeMacroDefType`

I guess having `computeMacroDefType` is the remnant from the times when
we considered an immediate possibility of having non-defdef macros, for
instance type macros, which would be TypeDefs.

These happy early days are gone, type macros have been long buried,
and the perspectives of extensions to the existing def macro scheme are
unclear. Therefore let’s have maximally precise types right away and then
think of generalization later on, once/if we get there.