nix-shell: Don't leave a temporary directory in /tmp behind
Move manpages around
nix-instantiate: Add a flag --expr / -E to read expressions from the command line

This is basically a shortcut for ‘echo 'expr...' | nix-instantiate -’.
Also supported by nix-build and nix-shell.
nix-instantiate: Rename --eval-only to --eval, --parse-only to --parse
nix-instantiate: Allow --dry-run as a synonym for --readonly-mode

--dry-run is more consistent with nix-env and nix-store.
nix-shell: Add --packages flag

This allows you to easily set up a build environment containing the
specified packages from Nixpkgs.  For example:

  $ nix-shell -p sqlite xorg.libX11 hello

will start a shell in which the given packages are present.