add concatMapM to base in control.monad

friend asked me to raise this previously https://ghc.haskell.org/trac/ghc/ticket/2042 seems like it iddn't happen last time because base lived outside of ghc? -- | The 'concatMapM' function generalizes 'concatMap' to arbitrary monads. concatMapM :: (Monad m) => (a -> m [b]) -> [a] -> m [b] concatMapM f xs = liftM concat (mapM f xs)