Add support for Sync argument

[dcchut]

This allows a Sync bound to be included on the future returned by the modified function:

#[async_recursion(Sync)]
async fn returned_future_is_send_and_sync() {}

As mentioned in this comment, a Sync bound on a Future is useless, since you can't do anything with a &Future. That being said, you may still want to store the resulting future in a struct, and you might want that struct to be Sync, so there are use-cases here.

Given that this is something that most people shouldn't need (nor use!), a Sync bound is not included by default.