use the constructor function that defstruct created, not make-instance when allocating wrapper instances

using make-instance breaks on Allegro CL 8.2

As far as I can tell, there are only two places where wrappers are created. But maybe I've missed something? Also, did I handle anonymous foreign types properly? It seems when defstruct is given an uninterned symbol, it uses the current package for the functions it defines, so I think using *package* is correct?

This is the last change required to get CL-SDL2 running on windows in ACL 8.2.