Add generalised "lookup" function

I think it would be very nice to have a default implementation of a generalised lookup function, such as lookupWith. The code I'm currently using is very simple and straight-forward:

lookupWith :: (a -> Bool) -> [(a, b)] -> Maybe b
lookupWith _ []          = Nothing
lookupWith p ((x, y):xs) | p x = Just y
                         | otherwise = lookupWith p xs

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Trac field	Value
Version	6.11
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Prelude
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