Eliminate some orphan-instance modules to speed up compilation.

I decided to just bite the bullet and give Data.Dynamic an .hi-boot
file, so I could remove GHC.Dynamic altogether, move its data types
into Data.Dynamic and hence prevent Data.Dynamic from being an orphan
module.  Furthermore, GHC.Dynamic wasn't GHC specific - its only
purpose in life was to prevent module loops, so having it at all was
artificial.