for every install there should be an uninstall

(Imported from [Trac #655](http://hackage.haskell.org/trac/hackage/ticket/655), reported by nr on 2010-04-07)

This is a feature request for a 'cabal uninstall' that will undo
all the effects of a 'cabal install'. This should include

• Unregistering the package with ghc-pkg
  
• Recovering the disk space
  
• Removing from indices and documentation
  
• Transitively uninstalling packages that were installed
  only to satisfy dependencies of this package, all of whose
  dependencies are dead.
  

That last bullet could be a bit of a problem, but the
first three should be straightforward I hope.