Up to now, the data constructor dictionary for class C as been called
":DC". But there is no reason for the colon to be at the front; indeed
it confuses the (simple-minded) pretty-printer for types.  So this
patch changes it to be "D:C".  This makes Core a lot easier to read.
Having a colon in the middle ensures that it can't clash with a user-written
data type.

Similarly I changed

  T:C 	   Data type corresponding a class dictionary (was :TC)
  D:C	   Data constructor for class dictionary (was :DC)

  NTCo:T   Coercion mapping from a newtype T to its representation type
		(was :CoT)

  TFCo:R   Coercion mapping from a data family to its respresentation type R
		(was :CoFR)

  Rn:T     The n'th respresentation data type for a data type T
		(was :RnT)


Do not merge to 6.10.

HEADS-UP: you'll need to recompile libraries from scratch.  

ROMAN: you could do the same for OccName.mkVectTyConOcc etc, if you wanted.