Fix Unusable Sink Implementation on scan (#2499)

The current Sink implementation on stream::Scan requires the stream and
state types are the same which doesn't practically allow that
implementation to be used. This adds a separate generic to the Sink
implementation for state.

Author: drewkett

Diff for: futures-util/src/stream/stream/scan.rs

@@ -118,11 +118,11 @@ where
 
 // Forwarding impl of Sink from the underlying stream
 #[cfg(feature = "sink")]
-impl<S, Fut, F, Item> Sink<Item> for Scan<S, S, Fut, F>
+impl<St, S, Fut, F, Item> Sink<Item> for Scan<St, S, Fut, F>
 where
-    S: Stream + Sink<Item>,
+    St: Stream + Sink<Item>,
 {
-    type Error = S::Error;
+    type Error = St::Error;
 
     delegate_sink!(stream, Item);
 }