net: flatten split mod (#1797)

The misc `split` types (`ReadHalf`, `WriteHalf`, `SendHalf`, `RecvHalf`)
are moved up a module and the `*::split` module is removed.

examples/connect.rs

@@ -115,12 +115,13 @@ mod tcp {
 }
 
 mod udp {
+    use tokio::net::udp::{RecvHalf, SendHalf};
+    use tokio::net::UdpSocket;
+
     use futures::{future, Sink, SinkExt, Stream, StreamExt};
-    use std::{error::Error, io, net::SocketAddr};
-    use tokio::net::udp::{
-        split::{UdpSocketRecvHalf, UdpSocketSendHalf},
-        UdpSocket,
-    };
+    use std::error::Error;
+    use std::io;
+    use std::net::SocketAddr;
 
     pub async fn connect(
         addr: &SocketAddr,
@@ -146,7 +147,7 @@ mod udp {
 
     async fn send(
         mut stdin: impl Stream<Item = Result<Vec<u8>, io::Error>> + Unpin,
-        writer: &mut UdpSocketSendHalf,
+        writer: &mut SendHalf,
     ) -> Result<(), io::Error> {
         while let Some(item) = stdin.next().await {
             let buf = item?;
@@ -158,7 +159,7 @@ mod udp {
 
     async fn recv(
         mut stdout: impl Sink<Vec<u8>, Error = io::Error> + Unpin,
-        reader: &mut UdpSocketRecvHalf,
+        reader: &mut RecvHalf,
     ) -> Result<(), io::Error> {
         loop {
             let mut buf = vec![0; 1024];