WIP: Move warnings from the desugarer into the typechecker

Don't review yet.

The goal here is to move all the warnings from the desugarer into the typechecker. As a principle, GHC warns users about source programs and so should have completed its warnings by the end of typechecking. This also makes -fno-code work better as it doesn't have to run the desugarer just to produce warnings.

The original motivation was to move the pattern match checker into the typechecker but I noticed that there were these other warnings as well so he tried to move those first to see if it was easily possible.