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/generate/src/module_trait.rs

@@ -89,8 +89,7 @@ pub fn define_module_trait(names: &Names, m: &Module, settings: &CodegenSettings
     });
 
     quote! {
-        use #rt::async_trait;
-        #[async_trait(?Send)]
+        #[#rt::async_trait]
         pub trait #traitname {
             #(#traitmethods)*
         }