Export the affinity API from Control.Concurrent: forkOn and friends.

Export the affinity API from Control.Concurrent: forkOn and friends.

forkOn             :: Int -> IO () -> IO ThreadId
forkOnWithUnmask   :: Int -> ((forall a . IO a -> IO a) -> IO ()) -> IO ThreadId
getNumCapabilities :: IO Int
threadCapability   :: ThreadId -> IO (Int, Bool)

Following discussion on the libraries list, I renamed forkOnIO to
forkOn.  In due course we might want to also rename forkIO to fork.

I left the Int argument as it is, it's quite useful to be able to
specify a number to be interpreted modulo the actual number of
processors.

I also used the term "capability" consistently.  It might not be the
best choice, but we have to pick something.