remove extra casts in C stubs
remove extra casts in C stubs

Ignore-this: 141ad50ecda6baffee4756bcc8dfbb8c

darcs-hash:20120703004614-c41ad-6ba2e326b8172c2b7515623b8e3a49460d6e0a2e

rgrinberg/lwt,c-cube/lwt,ocsigen/lwt,ocsigen/lwt,mirage/lwt,rgrinberg/lwt,mirage/lwt,rneswold/lwt,ocsigen/lwt,rgrinberg/lwt,gfxmonk/lwt,fdopen/lwt,c-cube/lwt,mfp/lwt,janestreet/lwt-async,mfp/lwt,janestreet/lwt-async,gfxmonk/lwt,rneswold/lwt,c-cube/lwt,fdopen/lwt,rgrinberg/lwt,rneswold/lwt