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/libstore/store-api.hh

@@ -140,9 +140,10 @@ struct BuildResult
 
 struct BasicDerivation;
 struct Derivation;
+struct FSAccessor;
 
 
-class Store
+class Store : public std::enable_shared_from_this<Store>
 {
 public:
 
@@ -314,6 +315,9 @@ public:
        remain. */
     virtual bool verifyStore(bool checkContents, bool repair) = 0;
 
+    /* Return an object to access files in the Nix store. */
+    virtual ref<FSAccessor> getFSAccessor() = 0;
+
     /* Utility functions. */
 
     /* Read a derivation, after ensuring its existence through
@@ -345,6 +349,12 @@ public:
 };
 
 
+class LocalFSStore : public Store
+{
+    ref<FSAccessor> getFSAccessor() override;
+};
+
+
 /* !!! These should be part of the store API, I guess. */
 
 /* Throw an exception if `path' is not directly in the Nix store. */