repository: extract Load/StoreJSONUnpacked

A Load/Store method for each data type is much clearer. As a result the
repository no longer needs a method to load / store json.

internal/restorer/restorer_test.go

@@ -137,7 +137,7 @@ func saveSnapshot(t testing.TB, repo restic.Repository, snapshot Snapshot) (*res
 	}
 
 	sn.Tree = &treeID
-	id, err := repo.SaveJSONUnpacked(ctx, restic.SnapshotFile, sn)
+	id, err := restic.SaveSnapshot(ctx, repo, sn)
 	if err != nil {
 		t.Fatal(err)
 	}