SI-7340 no longer allows to convert vampires to zombies

As eloquently elaborated and cleverly named by Travis Brown, macros
defined in structural types are useful:
http://meta.plasm.us/posts/2013/07/12/vampire-methods-for-structural-types/.

However, since such macros are on the intersection of a number of language
features, as usual, there are bugs.

Before this commit, subtyping checks between structural types didn't
distinguish regular methods structural types (zombies, as per Travis'
classification) and macros (vampires, again as per the same bestiary).