Refactor TcTyCon into its own type

TcTyCon is currently a constructor of TyCon. However, there are a number of places during type inference where we know statically that a TyCon should always be a TcTyCon. Indeed we have a type synonym type TcTyCon = TyCon for this purpose.

I think it would be better to move TcTyCon out to its own type and then embed that in TyCon as necessary. (If we ever split TcType from Type, we won't embed in TyCon.) This gives better static guarantees. With an {-# UNPACK #-}, I don't think there is any loss, at all.