Proposal: Add Control.Exception.allowInterrupt

[Simon Marlow]

Ticket:

   http://hackage.haskell.org/trac/ghc/ticket/4857

This proposal is to add the following function to Control.Exception:

-- | When invoked inside 'mask', this function allows a blocked
-- asynchronous exception to be raised, if one exists.  It is
-- equivalent to performing an interruptible operation (see
-- #interruptible#), but does not involve any actual blocking.
--
-- When called outside 'mask', or inside 'uninterruptibleMask', this
-- function has no effect.
allowInterrupt :: IO ()
allowInterrupt = unsafeUnmask $ return ()

The above is almost the entire implementation modulo imports, so I 
didn't bother attaching a patch to the ticket.  unsafeUnmask is not 
exported by Control.Exception, it is an internal function from GHC.IO.

Some discussion leading up to this can be found in 
http://hackage.haskell.org/trac/ghc/ticket/3837.

Discussion period: 3 weeks (until 12 Jan 2011)

Cheers,
	Simon