Proposal: Add default instances for Functor and Applicative

[Maciej Marcin Piechotka]

The problem of backward compatibility have been the main obstacle
against adopting Functor f => (Pointed f =>?) => Applicative f => Monad
f.

This proposition is to add following default instances[1]:

default fmap :: Applicative f => (a -> b) -> f a -> f b
f `fmap` m = pure f <*> m
default pure :: Monad f => a -> f a
pure = return
default (<*>) :: Monad f => f (a -> b) -> f a -> f b
(<*>) = liftM2 ($)

The proposition is intended as step towards implementing whole hierarchy
of Functor f => (Pointed f =>?) => Applicative f => Monad f[2]

Discussion period: 2 weeks

Regards

[1] It's using DefaultSuperclassInstances extentions:
http://hackage.haskell.org/trac/ghc/wiki/DefaultSuperclassInstances
[2] Trivial implementation is shown here:
http://thread.gmane.org/gmane.comp.lang.haskell.libraries/16196

I believe that in such case the Pointed instance comes at nearly zero
cost.
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