Add `+Sync` argument to mark the future as `Sync`

[rcelha]

Hey,

I ran into the issue where some of my futures must be Send + Sync, browsing the issue tracker I've found https://github.com/dtolnay/async-trait/issues/77 too.

Is this something you would be willing to accept contribution to? If that is the case, I'd love to discuss the best approach.

This PR is a draft/proposal on how we can solve this issue. I don´t particularly like my implementation tho, but once we agree on an API I can polish the implementation.

---

The API I am using here is as follows:

#[async_trait(+Sync)]
pub trait Trait {
    async fn f(&self);
}

struct A;

#[async_trait]
impl Trait for A {
    async fn f(&self) {}
}

---

Thanks

[Qqwy]

This PR/Fork saved me big time just now!

I wanted to iterate over a tree structure whose nodes are fetched asynchroniously from persistent storage. Needless to say, this results in quite the combination of nesting of Box, Future and trait objects of an iterator/stream-like trait.

Essentially, whenever you'd want to write something close to std::async_iterator::AsyncIterator or futures_core::stream::Stream you end up in a situation in which you are building a trait that:

• Has many default methods (for the iterator adaptors) that take either &self or &mut self, necessitating + Send + Sync bounds.
• Need to handle recursion to handle e.g. flat_map(), map().flatten(), etc.
• Therefore, any future you might want to use inside flat_map also needs to be Send + Sync.
• And thus futures in this situation really needs the Sync bound!

[rcelha]

I am glad it helped someone else as well @Qqwy !