#2309: containers: specialize functions that fail in a Monad to Maybe

[Twan van Laarhoven]

Ross Paterson wrote:
> The proposal is to change these signatures back, specializing them
> to Maybe.

To retain most of the convenience I would propose we use MonadPlus instead, 
which is arguably where fail should have been put in the first place (*).

Twan


(*) Actually in a superclass MonadZero, but let's work with what we have now.