[clippy] enable more lints, get rid of unwrap

src/main.rs

@@ -1,6 +1,6 @@
 use std::{
-    env,
     net::{IpAddr, SocketAddr},
+    num::NonZeroUsize,
     thread::available_parallelism,
 };
 
@@ -39,7 +39,7 @@ async fn main() -> std::io::Result<()> {
                 .prefer_utf8(true),
         )
     })
-    .workers(available_parallelism().unwrap().get())
+    .workers(available_parallelism().map_or(1, NonZeroUsize::get))
     .bind(sock)?
     .run()
     .await