Add Read and Write for Async TunInterface

Those features are implemented using AsyncFD. While write doesn't
require a mutable reference to self, read does.

Make Async Tun a feature

remove async tun from workspace

rename write/read to send/recv

tun/src/unix/linux/mod.rs

@@ -166,17 +166,11 @@ impl TunInterface {
         let socket = Socket::new(Domain::IPV6, Type::DGRAM, None)?;
         perform(socket.as_raw_fd())?
     }
-}
 
-impl Write for TunInterface {
     #[throws]
-    fn write(&mut self, buf: &[u8]) -> usize {
+    pub fn send(&self, buf: &[u8]) -> usize {
         self.socket.send(buf)?
     }
-
-    fn flush(&mut self) -> std::io::Result<()> {
-        Ok(())
-    }
 }
 
 #[cfg(test)]