* Get `nix-push' working again. It now uses Nix/Fix to create Nix

archives (using the package in corepkgs/nar).
* queryPathByHash -> expandHash, and it takes an argument specifying
  the target path (which may be empty).
* Install the core Fix packages in $prefix/share/fix.  TODO: bootstrap
  Nix and install Nix as a Fix package.

corepkgs/nar/nar.fix

@@ -0,0 +1,8 @@
+Function(["path", "name"],
+  Package(
+    [ ("name", Var("name"))
+    , ("build", Relative("nar/nar.sh"))
+    , ("path", Var("path"))
+    ]
+  )
+)

corepkgs/nar/nar.sh

@@ -0,0 +1,3 @@
+#! /bin/sh
+
+/tmp/nix/bin/nix --dump --file "$path" > $out || exit 1