Add instance IsString e => MonadFail (Either e) to Control.Monad.Fail

[Gurkenglas]

http://ircbrowse.net/browse/haskell?id=23946308&timestamp=1483313566#t1483313566 (irrelevant lines taken):

<libscott>  Anyone here use Control.Monad.Except?
<libscott>  kind of a drag that `fail` doesn't return a Left
<libscott>  also it doesn't export a utility function to convert an Either to an ExceptT
<Cale>  libscott: It's not a drag -- that allows the instance of Monad to apply to more than Either String
<Cale>  :t fail
<lambdabot> Monad m => String -> m a
<Cale>  If you tried to say that fail = Left
<Cale>  Then it only works for Either String, and not for other types
<Gurkenglas>  Then why don't we have MonadFail (Either String)?
<Gurkenglas>  fail's on track to leave Monad anyway
<Gurkenglas>  (Or rather IsString e => MonadFail (Either e))
<Cale>  Gurkenglas: Yes, that would be the sensible thing

[neongreen]

See https://ghc.haskell.org/trac/ghc/ticket/12160 and https://mail.haskell.org/pipermail/libraries/2016-August/027251.html. The previous proposal at libraries hasn't received much discussion, so I'll make a new proposal at some point (probably soon).