Added `DefaultMessageSender`, `DefaultMessageReceiver`, `DefaultMessageSenderReceiver` types, which function the same as `()` for the cases of the trait(s) that they implement.

Fortune for rsh's current commit: Half blessing − 半吉

src/message/serial.rs

@@ -24,6 +24,25 @@ pub trait MessageReceiver
 impl MessageSender for (){}
 impl MessageReceiver for (){}
 
+/// Identical to `()` in terms of the use of the MessageSender trait.
+#[derive(Debug, Clone, Copy, Default, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub struct DefaultMessageSender;
+
+impl MessageSender for DefaultMessageSender{}
+
+/// Identical to `()` in terms of the use of the MessageReceiver trait.
+#[derive(Debug, Clone, Copy, Default, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub struct DefaultMessageReceiver;
+
+impl MessageReceiver for DefaultMessageReceiver{}
+
+/// Identical to `()` in terms of the use of the MessageSender/Receiver traits.
+#[derive(Debug, Clone, Copy, Default, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub struct DefaultMessageSenderReceiver;
+
+impl MessageSender for DefaultMessageSenderReceiver{}
+impl MessageReceiver for DefaultMessageSenderReceiver{}
+
 #[derive(Debug)]
 pub(super) struct WriteCounter<W:?Sized>(pub usize, pub W);