Warn when a class implements a method that is to be removed in the future.
A more general mechanism of achieving this would be by introducing e.g. a REMOVED pragma, but since the 8.0 release is somewhat close the hardcoded ad-hoc version will have to do for the time being.
- Write tests
- Fix warnings in GHC
- Flesh out warnings, link to wiki article with details about fixing them (e.g. don't implement *> in terms of >>)
- Figure out how to be two versions bootstrap compatible