wiggle::async_trait is defined as async_trait::async_trait(?Send)

async methods used by wiggle currently need to Not have the Send
constraint, so rather than make all use sites pass the argument
to the re-exported async_trait macro, define a new macro that
applies the argument.

crates/wiggle/tests/atoms_async.rs

@@ -14,7 +14,7 @@ wiggle::from_witx!({
 
 impl_errno!(types::Errno);
 
-#[wiggle::async_trait(?Send)]
+#[wiggle::async_trait]
 impl<'a> atoms::Atoms for WasiCtx<'a> {
     async fn int_float_args(&self, an_int: u32, an_float: f32) -> Result<(), types::Errno> {
         println!("INT FLOAT ARGS: {} {}", an_int, an_float);