Update struct references to CFFI version 11 syntax

Recent CFFI releases have deprecated "bare refences to struct types". According to the warning references to a-struct should be changed to (:struct a-struct) or (:pointer (:struct a-struct)). I'm new to CFFI which makes me hesitant to make any guarantees about this patch, but on my system it gets rid of the CFFI warnings without hindering the functioning of the library.