Add DeltaObservable class

src/lib/Qd/Observable/Delta.hs

+module Qd.Observable.Delta where
+
+import Qd.Observable
+import Qd.Prelude
+
+import Conduit
+import qualified Data.HashMap.Strict as HM
+import Data.Binary (Binary(..))
+
+data Delta k v = Reset (HM.HashMap k v) | Add k v | Change k v | Remove k
+  deriving (Eq, Show, Generic)
+instance (Binary k, Binary v) => Binary (Delta k v) where
+  get = undefined
+  put = undefined
+
+class Observable (HM.HashMap k v) o => DeltaObservable k v o | o -> k, o -> v where
+  subscribeDelta :: o -> (Delta k v -> IO ()) -> IO SubscriptionHandle
+  subscribeDelta = undefined
+  subscribeDeltaC :: o -> ConduitT () (Delta k v) IO ()
+  subscribeDeltaC = undefined
+  {-# MINIMAL subscribeDelta | subscribeDeltaC #-}