This should be more robust and also plays better with
cross-compilation---it uses the host name, instead of using the build
name.

  read the manifest just to check the version and print the number of
  paths.  This makes nix-pull very fast for the cached cache (speeding
  up nixos-rebuild without the ‘--no-pull’ or ‘--fast’ options).

  disk space, and, since they're typically only decompressed once (to
  fill the manifest cache), doesn't make things slower.

  DB.  This speeds up creating the cache from 16.1s to 7.9s on my
  system.

  intermittent problems are gone by now.  WAL mode is preferrable
  because it does way fewer fsyncs.

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  (reported by Silvio Frischknecht).

  a string.  This happens in the NixOS option system.
* Remove a bogus comparison of a unsigned integer with -1.

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  the Nix:: namespace.

  bindings to be used in Nix's own Perl scripts.

  The only downside is that Perl XS and Automake/libtool don't really
  like each other, so building is a bit tricky.

  causing a deadlock.

  the contents of any of the given store paths have been modified.
  E.g.

    $ nix-store --verify-path $(nix-store -qR /var/run/current-system)
    path `/nix/store/m2smyiwbxidlprfxfz4rjlvz2c3mg58y-etc' was modified! expected hash `fc87e271c5fdf179b47939b08ad13440493805584b35e3014109d04d8436e7b8', got `20f1a47281b3c0cbe299ce47ad5ca7340b20ab34246426915fce0ee9116483aa'

  All paths are checked; the exit code is 1 if any path has been
  modified, 0 otherwise.

  This should also fix:

    nix-instantiate: ./../boost/shared_ptr.hpp:254: T* boost::shared_ptr<T>::operator->() const [with T = nix::StoreAPI]: Assertion `px != 0' failed.

  which was caused by hashDerivationModulo() calling the ‘store’
  object (during store upgrades) before openStore() assigned it.

  not really supposed to use according to the DBD::SQLite docs, and
  fails on some systems (e.g. http://hydra.nixos.org/build/1246662).

* Drop some old Fedora/Debian/Ubuntu releases.

  prevents files from being evaluated and stored as values multiple
  times.  For instance, evaluation of the ‘system’ attribute in NixOS
  causes ‘nixpkgs/pkgs/lib/lists.nix’ to be evaluated 2019 times.

  Caching gives a modest speedup and a decent memory footprint
  reduction (e.g., from 1.44s to 1.28s, and from 81 MiB to 59 MiB with
  GC_INITIAL_HEAP_SIZE=100000 on my system).