Add a command "nix-build --run-env" to reproduce the environment of a derivation

This command builds or fetches all dependencies of the given
derivation, then starts a shell with the environment variables from
the derivation.  This shell also sources $stdenv/setup to initialise
the environment further.

The current directory is not changed.  Thus this is a convenient way
to reproduce a build environment in an existing working tree.

Existing environment variables are left untouched (unless the
derivation overrides them).  As a special hack, the original value of
$PATH is appended to the $PATH produced by $stdenv/setup.

Example session:

$ nix-build --run-env '<nixpkgs>' -A xterm
(the dependencies of xterm are built/fetched...)
$ tar xf $src
$ ./configure
$ make
$ emacs
(... hack source ...)
$ make
$ ./xterm

perl/lib/Nix/Store.pm

@@ -16,6 +16,7 @@ our @EXPORT = qw(
     topoSortPaths computeFSClosure followLinksToStorePath exportPaths
     hashPath hashFile hashString
     addToStore makeFixedOutputPath
+    derivationFromPath
 );
 
 our $VERSION = '0.15';