Use raw fork instead of forkIO

[harendra-kumar]

Since we create threads quite often it may be helpful to use raw fork instead of forkIO. Something like this:

-- A version of forkIO that does not include the outer exception
-- handler: saves a bit of time when we will be installing our own
-- exception handler.
{-# INLINE rawForkIO #-}
rawForkIO :: IO () -> IO ThreadId
rawForkIO action = IO $ \ s ->
   case (fork# action s) of (# s1, tid #) -> (# s1, ThreadId tid #)

[agocorona]

If this is faster, It would be worth the pain