Use s.parse(), not u16::from_str_radix(s,10)

Clippy started warning on this with Rust 1.52.  I'm fairly sure that
when I wrote this code I was worried that <u16 as FromStr>::parse()
might allow hex or something.  But fortunately, it doesn't work that
way, so I don't need to be paranoid here.

tor-cell/src/relaycell/msg.rs

@@ -260,7 +260,7 @@ impl Body for Begin {
         let port = std::str::from_utf8(port)
             .map_err(|_| Error::BadMessage("port in begin cell not utf8"))?;
 
-        let port = u16::from_str_radix(port, 10)
+        let port = port.parse()
             .map_err(|_| Error::BadMessage("port in begin cell not a valid port"))?;
 
         Ok(Begin {