Rename Untouchables to TcLevel

This is a long-overdue renaming
   Untouchables  -->   TcLevel
It is renaming only; no change in functionality.

We really wanted to get this done before the 7.10 fork.
Test Trac #4921
Minor refactoring of Edward's recent orphans patch (Trac #2182)

This patch is all small stuff
  - Move VisibleOrphanModules from Module to InstEnv (with the other orphan stuff)
  - Move Notes about orphans from IfaceSyn to InstEnv (ditto)
  - Make use of the record field names in InstEnvs
Fix another bug in deriving( Data ) for data families; Trac #4896

If we have

   data family D a
   data instance D (a,b,c) = ... deriving( Data )

then we want to generate

   instance ... => Data (D (a,b,c)) where
     ...
     dataCast1 x = gcast1 x

The "1" here comes from the kind of D.  But the kind of the
*representation* TyCon is

   data Drep a b c = ....

ie Drep :: * -> * -> * -> *

So we must look for the *family* TyCon in this (rather horrible)
dataCast1 / dataCast2 binding.