* Re-enable support for substitutes in the normaliser.

* A better substitute mechanism.

  Instead of generating a store expression for each store path for
  which we have a substitute, we can have a single store expression
  that builds a generic program that is invoked to build the desired
  store path, which is passed as an argument.

  This means that operations like `nix-pull' only produce O(1) files
  instead of O(N) files in the store when registering N substitutes.
  (It consumes O(N) database storage, of course, but that's not a
  performance problem).

* Added a test for the substitute mechanism.
  
* `nix-store --substitute' reads the substitutes from standard input,
  instead of from the command line.  This prevents us from running
  into the kernel's limit on command line length.

tests/substitutes.sh

@@ -0,0 +1,25 @@
+# Instantiate.
+storeExpr=$($TOP/src/nix-instantiate/nix-instantiate substitutes.nix)
+echo "store expr is $storeExpr"
+
+# Find the output path.
+outPath=$($TOP/src/nix-store/nix-store -qvvvvv "$storeExpr")
+echo "output path is $outPath"
+
+# Instantiate the substitute program.
+subExpr=$($TOP/src/nix-instantiate/nix-instantiate substituter.nix)
+echo "store expr is $subExpr"
+
+# Register a fake successor, and a substitute for it.
+suc=$NIX_STORE_DIR/aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa-s.store
+(echo $suc && echo $subExpr && echo "/substituter" && echo 3 && echo $outPath && echo Hallo && echo Wereld) | $TOP/src/nix-store/nix-store --substitute
+$TOP/src/nix-store/nix-store --successor $storeExpr $suc
+
+# Register a substitute for the output path.
+(echo $outPath && echo $subExpr && echo "/substituter" && echo 3 && echo $outPath && echo Hallo && echo Wereld) | $TOP/src/nix-store/nix-store --substitute
+
+
+$TOP/src/nix-store/nix-store -rvvvvv "$storeExpr"
+
+text=$(cat "$outPath"/hello)
+if test "$text" != "Hallo Wereld"; then exit 1; fi