NARs.  Fixes the impurity of nix-push (NIX-21).
* Better help.

* Restore the mtime on modified directories.

  all identitical files in the Nix store to each other.  (Previously
  it only computed the size that would be saved by doing so.)

  package duplication present in (e.g.) a profile.  It shows the
  number of instances of each package in a closure, along with the
  size in bytes of each instance as well as the "waste" (the
  difference between the sum of the sizes of all instances and the
  average size).

  $ ./show-duplication.pl /nix/var/nix/profiles/default
  gcc 11
    3.3.6 19293318
    3.4.4 21425257
    ...
    average 14942970, waste 149429707
  coreutils 6
  ...
  average package duplication 1.87628865979381, total size 3486330471, total waste 1335324237, 38.3017114443825% wasted

  This utility is useful for measuring the cost in terms of disk space
  of the Nix approach.

  --delete-generations: lock the profile to prevent (extremely
  unlikely) race conditions.

  Nix-env failed to call addPermRoot(), which is necessary to safely
  add a new root.  So if nix-env started after and finished before the
  garbage collector, the user environment (plus all other new stuff)
  it built might be garbage collected, leading to a dangling symlink
  chain in ~/.nix-profile...

* Be more explicit if we block on the GC lock ("waiting for the big
  garbage collector lock...").

* Don't loop trying to create a new generation.  It's not necessary
  anymore since profiles are locked nowadays.

* More follow restrictions on layout.

  old generations of *all* profiles in /nix/var/nix/profiles, then
  runs the garbage collector.  Quick way to get rid of all old stuff.
  Of course, one cannot roll back to earlier points in time after
  this.

* Install libexpr header files.

  marshalling code.

* Optimise header file usage a bit.
* Compile the parser as C++.

  and returns its path.  This can be used to (for instance) write
  builders inside a Nix expression, e.g.,

  stdenv.mkDerivation {
    builder = "
      source $stdenv/setup
      ...
    ";
    ...
  }

  bad flex doesn't have lexical restrictions, the current solution
  isn't quite right...)

  evaluate its arguments.

  a warning.

  derivation attributes to flatten them into strings.  This is
  possible since string can nowadays be wrapped in contexts that
  describe the derivations/sources referenced by the evaluation of the
  string.