Major refactoring of Data/Generics.
This also affects the compiler (because of deriving issues).
This is an intermediate commit.
The library is supposed to compile fine.
But the deriving stuff for Data needs to be revised.
The testsuite for Data/Generics will not pass.
gread is broken at the moment.
So it is strongly recommended not to cvs upd for a few hours or a day.

Detailed description of changes:
- Split up Data/Dynamic into Data/Typeable and Data/Dynamic.
  (This makes clear what part is about TypeReps and cast vs.
   dynamics. The latter is not needed by Data/Generics.)
- Renamed Data/include/Dynamic.h -> Typeable.h to end confusion.
- Split up Data/Generics.hs in a set of modules.
- Revised class Data:
  - Got rid of gunfold but added fromConstr as more primtive one
  - Revised representations of constructors
  - Revised treatment of primitive types
- Revised type of gmapQ; preserved old gmapQ as gmapL
- Added a module Data/Types.hs for treatment of types as values.

This is going somewhere.