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/fs-accessor.hh

@@ -0,0 +1,30 @@
+#pragma once
+
+#include "types.hh"
+
+namespace nix {
+
+/* An abstract class for accessing a filesystem-like structure, such
+   as a (possibly remote) Nix store or the contents of a NAR file. */
+class FSAccessor
+{
+public:
+    enum Type { tMissing, tRegular, tSymlink, tDirectory };
+
+    struct Stat
+    {
+        Type type;
+        uint64_t fileSize; // regular files only
+        bool isExecutable; // regular files only
+    };
+
+    virtual Stat stat(const Path & path) = 0;
+
+    virtual StringSet readDirectory(const Path & path) = 0;
+
+    virtual std::string readFile(const Path & path) = 0;
+
+    virtual std::string readLink(const Path & path) = 0;
+};
+
+}