Remove unnecessary constraints from MonadComprehensions and ParallelListComp

Many parts of MonadComprehensions don't actually require monads instance, the following could do with a Functor constraint

fmapM :: Monad m => (a -> b) -> m a -> m b
fmapM f xs = [ f x | x <- xs ]

and I don't see any reason why the class MonadZip (from Control.Monad.Zip) requires a Monad constraint rather a Functor constraint:

class Functor f => FunctorZip f where
    fzip :: f a -> f b -> f (a,b)
    fzip = fzipWith (,)

    fzipWith :: (a -> b -> c) -> f a -> f b -> f c
    fzipWith f fa fb = fmap (uncurry f) (fzip fa fb)

    funzip :: f (a,b) -> (f a, f b)
    funzip fab = (fmap fst fab, fmap snd fab)

with the laws

    fmap (f *** g) (fzip fa fb) = fzip (fmap f fa) (fmap g fb)

    fmap (const ()) fa = fmap (const ()) fb
==> funzip (fzip fa fb) = (fa, fb)

Same with Applicative (see ApplicativeDo):

liftA2 :: Applicative f => (a -> b -> c) -> f a -> f b -> f c
liftA2 f a1 a2 = [ f x1 x2 | x1 <- a1, x2 <- a2 ]

The reason I bring this up is because I'm writing a DSL that uses length-indexed vectors whose Functor and FunctorZip instances are trivial but whose Monad instance is complicated and not need.

This proposal shares a similar rationale as ApplicativeDo.

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Trac field	Value
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Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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