Adding a world file to cabal-install

[Peter Robinson]

I've attached a patch to the ticket [1] for world-file support.

Every user-requested package is recorded, without implied
dependencies. A world file entry consists of the package name, the
requested version, and user flags (if any). Recording these flags for
each package provides functionality similar to the USE flags in
Gentoo. On compiler upgrade these packages can be reinstalled by:

# cabal install world

It is also possible to upgrade all packages in the world file. This
might come in handy as an alternative to the currently disabled
global-upgrade:

# cabal upgrade world

Peter

[1] http://hackage.haskell.org/trac/hackage/ticket/199