Add default MonadPlus or Alternative method to MonadFail

The documentation for Control.Monad.Fail notes

If your Monad is also MonadPlus, a popular definition is

...

fail _ = mzero

This could be made a default definition for MonadPlus or Alternative

class Monad m => MonadFail m where fail :: String -> m a fail = const empty default fail :: Alternative m => String -> m a

This simplifies instances like [] and Maybe

instance MonadFail [] instance MonadFail Maybe