NarIndexer: Implement skip

This improves the performance of parseNarListing, which is used by commands
like `nix nar ls` when the underlying source allows cheap seeks (like StringSource
or FdSource that does lseek).

For `nix nar ls` of a NAR for linux source tarball this cuts down the runtime almost
in half (from 300ms -> 175ms).

src/libutil/nar-listing.cc

@@ -112,6 +112,12 @@ NarListing parseNarListing(Source & source)
             pos += n;
             return n;
         }
+
+        void skip(size_t len) override
+        {
+            source.skip(len);
+            pos += len;
+        }
     };
 
     NarIndexer indexer(source);