More consistent poll_ready usage

The `#[inline]`s probably aren't necessary but are consistent with other
parts of the code and tower in general.

src/handler/into_service.rs

@@ -62,6 +62,7 @@ where
     type Error = Infallible;
     type Future = super::future::IntoServiceFuture;
 
+    #[inline]
     fn poll_ready(&mut self, _cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
         // `IntoService` can only be constructed from async functions which are always ready, or from
         // `Layered` which bufferes in `<Layered as Handler>::call` and is therefore also always