* 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.

src/nix-env/main.cc

@@ -499,7 +499,7 @@ static void opQuery(Globals & globals,
                 installedPaths.insert(i->second.outPath);
             
             for (DrvInfoList::iterator i = drvs2.begin(); i != drvs2.end(); ++i) {
-                Paths subs = querySubstitutes(i->drvPath);
+                Substitutes subs = querySubstitutes(i->drvPath);
                 cout << format("%1%%2%%3% %4%\n")
                     % (installedPaths.find(i->outPath)
                         != installedPaths.end() ? 'I' : '-')