xmpp-parsers: Reduce component handshake allocations

This function always appends to an existing string, so by passing it a
String directly we can avoid superfluous allocations.

Also tokio-xmpp was doing a bunch of &str to String for no reason around
there, let’s remove that too.

tokio-xmpp/src/component/mod.rs

@@ -87,7 +87,6 @@ impl<C: ServerConnector> Component<C> {
         timeouts: Timeouts,
     ) -> Result<Self, Error> {
         let jid = Jid::from_str(jid)?;
-        let password = password.to_owned();
         let stream = component_login(connector, jid.clone(), password, timeouts).await?;
         Ok(Component { jid, stream })
     }