Add NAR / Store accessor abstraction

This is primary to allow hydra-queue-runner to extract files like
"nix-support/hydra-build-products" from NARs in binary caches.

src/libutil/archive.hh

@@ -57,6 +57,7 @@ void dumpPath(const Path & path, Sink & sink,
 
 void dumpString(const std::string & s, Sink & sink);
 
+/* FIXME: fix this API, it sucks. */
 struct ParseSink
 {
     virtual void createDirectory(const Path & path) { };