Use structure specification (:struct foo) in defcstruct expansion

Modify #'generate-struct-accessors so that when generating accessors,
the structure is referred to with the (:struct foo) syntax, rather
than just 'foo.  While such a reference usually generates only a style
warning, if parse-deprecated-struct-type hasn't been called yet, as is
the case for slot defintions in the defcstruct expansion, it results
in an error.  This problem prevented shuffletron from compiling.