message: attributes: fix async fn not moving args into body
  This fixes compilation errors with `#[instrument]`ed `async fn`s on the
  latest nightly. The generated `async` block was not `async move`, so the
  function's attributes were not moved into it.
diff: @@ -170,7 +170,7 @@ pub fn instrument(args: TokenStream, item: TokenStream) -> TokenStream {
  let await_kwd = syn::Ident::new("await", block.span());
  quote_spanned! {block.span()=>
  tracing_futures::Instrument::instrument(
  - #async_kwd { #block },
  + #async_kwd move { #block },
  __tracing_attr_span
  )
  .#await_kwd
  