Feature request: Collections from the standard library

First off, **THANK YOU** for making this crate! The `Future`-aware synchronization primitives alone are worth their weight in gold! :grinning:

As for my request, is it possible to add in concurrent versions of the [standard collections](https://doc.rust-lang.org/std/collections/index.html)? I'm aware of crates like [`dash-map`](https://crates.io/crates/dashmap), but since `async-std` is supposed to replace `sync-std`, it feels like it would make sense for it to have collections in here.