…s across await points (bytecodealliance#9217)

* Use `tracing::Instrument` in generated bindings to avoid holding spans
across await points

Previously, if we had a WIT file with a function

async-fn: func();

then the generated code in `add_to_linker_get_host` would look like
this (if tracing is enabled and the function is async)

inst.func_wrap_async(
    "my-func",
    move |mut caller: wasmtime::StoreContextMut<'_, T>, (): ()| {
        wasmtime::component::__internal::Box::new(async move {
            let span = tracing::span!(
                tracing::Level::TRACE,
                "wit-bindgen import",
                module = "test",
                function = "my-func",
            );
            let _enter = span.enter();
            tracing::event!(tracing::Level::TRACE, "call");
            let host = &mut host_getter(caller.data_mut());
            let r = Host::my_func(host).await;
            tracing::event!(
                tracing::Level::TRACE,
                result = tracing::field::debug(&r),
                "return"
            );
            Ok(r)
        })
    },
)?;

This keeps the tracing span active, even when the resulting future is
suspended. The end result is that other unrelated tokio tasks running on
the same thread in the meantime will be shown as executing within the
`wit-bindgen import` span.

This commit changes the codegen to instead generate

inst.func_wrap_async(
    "async-fn",
    move |mut caller: wasmtime::StoreContextMut<'_, T>, (): ()| {
        use tracing::Instrument;

        let span = tracing::span!(
            tracing::Level::TRACE,
            "wit-bindgen import",
            module = "test",
            function = "async-fn",
        );
        wasmtime::component::__internal::Box::new(
            async move {
                tracing::event!(tracing::Level::TRACE, "call");
                let host = &mut host_getter(caller.data_mut());
                let r = Host::async_fn(host).await;
                tracing::event!(
                    tracing::Level::TRACE,
                    result = tracing::field::debug(&r),
                    "return"
                );
                Ok(r)
            }
            .instrument(span),
        )
    },
)?;

Here, `tracing::Instrument` takes care of entering the span when the
future is polled and exiting it when it is suspended.

Fixes bytecodealliance#9210

* Bless expanded macro outputs