add incremental hashing interface to Backend

As yet unused.

Backend.External could perhaps implement it too, although that would
involve sending chunks of data to it via a pipe or something, so likely
to be slow.

Backend/URL.hs

@@ -23,6 +23,7 @@ backend = Backend
 	{ backendVariety = URLKey
 	, genKey = Nothing
 	, verifyKeyContent = Nothing
+	, verifyKeyContentIncrementally = Nothing
 	, canUpgradeKey = Nothing
 	, fastMigrate = Nothing
 	-- The content of an url can change at any time, so URL keys are