The module DisplayView depends on ObjectTypes and vice-versa.
To break this circularity, we create a DisplayView.hi-boot 
file and an ObjectTypes.hi-boot file.  These files are themselves produced by 
compiling the files ObjectTypes.boot.hs and DisplayTypes.boot.hs, and moving 
the  resulting .hi files, or for ghc5.03 onwards, generating files in .hi-boot
format from the .hi format using --show-iface.