Remove MonadFail (ST s) instance

Sun Nov 24 17:35:00 UTC 2019

On Sun, 24 Nov 2019, Dan Burton wrote:

> We're talking about an instance in `base`, so if there's no other way, it could at least be a flag akin to
> -Wmissing-monadfail-instances that gets included in -Wcompat.
> But relatedly: should there be a way to deprecate instances? I would say yes.

Sounds like another application of instance warnings that we already 
talked about:
    https://gitlab.haskell.org/ghc/ghc/issues/11796