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

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.