Use serialize_entry instead of serialize_key + serialize_value when s…

…erialize flatten newtype enum variant

Serializers that reimplements serialize_entry will get benefits from that

serde/src/private/ser.rs

@@ -1125,8 +1125,7 @@ where
  where
  T: ?Sized + Serialize,
  {
- tri!(self.0.serialize_key(variant));
- self.0.serialize_value(value)
+ self.0.serialize_entry(variant, value)
  }
 
  fn serialize_seq(self, _: Option<usize>) -> Result<Self::SerializeSeq, Self::Error> {