* Fixed the register script.

  packages (i.e., the packages that should appear in the user's $PATH,
  and so on).  Based on this list, the script nix-populate creates a
  hierarchy of symlinks to the relevant files in those packages (e.g.,
  for pkg/bin and pkg/lib).  

  A nice property of nix-populate is that on each run it creates a
  *new* tree, rather than updating the old one.  It then atomically
  switches over to the new tree.  This allows atomic upgrades or
  rollbacks on the set of activated packages.

* Use MD5::Digest.

* Command `nix ensure' which is like `nix getpkg' except that if the
  has refers to a run action it will just ensure that the imports are
  there.
* Command `nix closure' to print out the closure of the set of
  descriptors under the import relation, starting at a set of roots.
  This can be used for garbage collection (e.g., given a list of
  `activated' packages, we can delete all packages not reachable from
  those).
* Command `nix graph' to print out a Dot graph of the dependency
  graph.
* `nix-addroot' adds a root for the (unimplemented) garbage collector.

  the build or run action should be perfomed on.  This ensures that
  descriptors have different hashes on different platforms.

* Added descriptors for gtkspell and its support package pspell.
  Gtkspell is an optional dependency of Pan, so we should add the
  ability to nix-instantiate to instantiate variants of a package
  based on a selection of features.

  action.  Run actions are described by uniquely hashed descriptors,
  just like build actions.  Therefore run actions can have
  dependencies, but these need not be the same as the build time
  dependencies (e.g., at runtime we can link against a different
  version of a dynamic library).  Example:

    nix run 31d6bf4c171282367065e0deecd7c579

  will run the Pan 0.13.91 newsreader with gtkspell support.

  dependencies.

  descriptor templates under the import relation.  I.e., we can now
  say:

    nix-instantiate outdir foo.nix

  which will create descriptors for foo.nix and all imported packages
  in outdir/.

  files) are now referenced using their cryptographic hashes.  

  This ensures that if two package descriptors have the same contents,
  then they describe the same package.  This property is not as
  trivial as it sounds: generally import relations cause this property
  not to hold w.r.t. temporality.  But since imports also use hashes
  to reference other packages, equality follows by induction.