Add Control.Monad.forM and forM_

Add Control.Monad.forM and forM_

flip mapM_ is more and more common, I find. Several suggestions have
been made to add this, as foreach or something similar. This patch
does just that:

    forM  :: (Monad m) => [a] -> (a -> m b) -> m [b]
    forM_ :: (Monad m) => [a] -> (a -> m b) -> m ()

So we can write:

    Prelude Control.Monad> forM_ [1..4] $ \x -> print x
    1
    2
    3
    4