runProcess/ createPipe

If Posix.runProcess really is supposed "our candidate for the high-level OS-independent primitive" (see documentation), it would help if there was a documented way of using it with pipes. The problem is that runProcess takes handles as arguments, while the obvious function, createPipe, returns two Fds. There seems to be no _documented_ way of converting an Fd to a Handle. I suggest therefore that either there be a createPipeHandle :: IO (Handle,Handle) (which would be nice, because it might avoid bothering the operating system completely, and would makes sense on non-Posix systems). Or else, the existing Posix.fdToHandle fdToHandle :: Fd -> IO Handle should be documented.