TypeChecker plugins and the TcS monad; given constraints in GHC HEAD

[Christiaan Baaij]

> Beyond the changes in Phab:D909, the only thing I'm still using
> unsafeTcPluginTcM for is newUnique (in order to generate new skolem
> variables), which could easily be added to the TcPluginM interface as
> well. Anyone need anything else?

No, In my own type-checker plugins I only use unsafeTcPluginTcM for TcmType.newEvVar, which has now been added to the TcPluginM in Phab:D909.