[Cython] PATCH: use function pointer arguments in import/export api functions

[Stefan Behnel]

Lisandro Dalcin wrote:
> Just to prevent this:
> 
> warning: ISO C forbids conversion of function pointer to object pointer type
> 
> - Please note that I've changed the names of some local variables to
> make {Import|Export}Function implementations more symmetric.

Fine with me. If this makes things more portable, and it doesn't look like
it breaks anything - just go and push.

BTW, it's good that that you split the patch into two.

Stefan