add concatMapM to base in control.monad

[Carter Schonwald]

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)
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