Swap packed read/write ends of futures/streams (#1271)

Accounts for WebAssembly/component-model#497

Author: alexcrichton

crates/guest-rust/rt/src/async_support/future_support.rs

@@ -166,8 +166,8 @@ pub unsafe fn future_new<T>(
 ) -> (FutureWriter<T>, FutureReader<T>) {
     unsafe {
         let handles = (vtable.new)();
-        let writer = handles as u32;
-        let reader = (handles >> 32) as u32;
+        let reader = handles as u32;
+        let writer = (handles >> 32) as u32;
         rtdebug!("future.new() = [{writer}, {reader}]");
         (
             FutureWriter::new(writer, vtable),

crates/guest-rust/rt/src/async_support/stream_support.rs

@@ -71,8 +71,8 @@ pub unsafe fn stream_new<T>(
 ) -> (StreamWriter<T>, StreamReader<T>) {
     unsafe {
         let handles = (vtable.new)();
-        let writer = handles as u32;
-        let reader = (handles >> 32) as u32;
+        let reader = handles as u32;
+        let writer = (handles >> 32) as u32;
         rtdebug!("stream.new() = [{writer}, {reader}]");
         (
             StreamWriter::new(writer, vtable),