#4159: move Monad and MonadFix instances for Either from mtl to base

[Evan Laforge]

> A reasonable case can be made for the existence of a monad that does
> something useful with fail. I don't think that that monad should be Either.

I don't mean to further derail the proposal, which I think is a worthy
one, but what's the feeling on a complete deprecation of 'fail' and
making 'pat <- ...' the same as 'let pat = ...'?  I've never used it,
not because I've never come across a case where it would be useful,
but that I don't trust to remember the behaviour of 'fail' for this
particular monad and even if I do, maybe my reader doesn't.

So I prefer 'case xyz of { pat -> mzero/throw/whatever }'.  It's more
wordy but clearer.  Are there uses out there that really rely on the
conciseness afforded by refutable patterns?