Warn about implicit kind variables with -Wcompat

According to an accepted proposal https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0024-no-kind-vars.rst:

With -Wcompat, warn if a kind variable is brought into scope implicitly in a type with an explicit forall. This applies to type signatures and to other contexts that allow a forall with the forall-or-nothing rule in effect (for example, class instances).

Creating ticket just for the record. Implementation coming soon.