Fix warnings and deny warnings on CI (#41)

Author: taiki-e

.github/workflows/main.yml

@@ -1,6 +1,10 @@
 on: [push, pull_request]
 
-name: Continuous integration
+name: CI
+
+env:
+  RUSTFLAGS: -Dwarnings
+  RUST_BACKTRACE: 1
 
 jobs:
   check:

async-stream/examples/tcp_accept.rs

@@ -5,7 +5,7 @@ use tokio::net::TcpListener;
 
 #[tokio::main]
 async fn main() {
-    let mut listener = TcpListener::bind("127.0.0.1:0").await.unwrap();
+    let listener = TcpListener::bind("127.0.0.1:0").await.unwrap();
 
     let incoming = stream! {
         loop {

async-stream/tests/stream.rs

@@ -101,7 +101,7 @@ async fn return_stream() {
 
 #[tokio::test]
 async fn consume_channel() {
-    let (mut tx, mut rx) = mpsc::channel(10);
+    let (tx, mut rx) = mpsc::channel(10);
 
     let s = stream! {
         while let Some(v) = rx.recv().await {