Increase type sharing

Fixes #20541 by making mkTyConApp do more sharing of types.
In particular, replace

* BoxedRep Lifted    ==>  LiftedRep
* BoxedRep Unlifted  ==>  UnliftedRep
* TupleRep '[]       ==>  ZeroBitRep
* TYPE ZeroBitRep    ==>  ZeroBitType

In each case, the thing on the right is a type synonym
for the thing on the left, declared in ghc-prim:GHC.Types.
See Note [Using synonyms to compress types] in GHC.Core.Type.

The synonyms for ZeroBitRep and ZeroBitType are new, but absolutely
in the same spirit as the other ones.   (These synonyms are mainly
for internal use, though the programmer can use them too.)

I also renamed GHC.Core.Ty.Rep.isVoidTy to isZeroBitTy, to be
compatible with the "zero-bit" nomenclature above.  See discussion
on !6806.

There is a tricky wrinkle: see GHC.Core.Types
  Note [Care using synonyms to compress types]

Compiler allocation decreases by up to 0.8%.