macros: improve try_join macro hygiene

The macro was previously relying on the library user not having any
other `Ok` symbol in scope that didn't align with the expected variant
in the standard library's `Result` type.

Now, a full path to the standard library's `Ok` variant is used instead,
which should protect the macro from compile errors that actually require
looking at the underlying implementation to solve.

Author: dybucc

tokio/src/macros/support.rs

@@ -29,4 +29,5 @@ cfg_macros! {
 
 pub use std::future::{Future, IntoFuture};
 pub use std::pin::Pin;
+pub use std::result::Result;
 pub use std::task::{Context, Poll};

tokio/src/macros/try_join.rs

@@ -241,7 +241,7 @@ doc! {macro_rules! try_join {
             if is_pending {
                 $crate::macros::support::Poll::Pending
             } else {
-                $crate::macros::support::Poll::Ready(Ok(($({
+                $crate::macros::support::Poll::Ready($crate::macros::support::Result::Ok(($({
                     // Extract the future for this branch from the tuple.
                     let ( $($skip,)* fut, .. ) = &mut futures;