optimize character reading/writing in Lwt_io
optimize character reading/writing in Lwt_io

Ignore-this: 16af19b9b6ae74f6df5bf4c0fa8ff06

Bypass the locking/unlocking phase if we can.

darcs-hash:20100106103443-c41ad-1f7a9057724da138024c1ef0a8f377004d9874a4

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