Release 0.3.15

[taiki-e]

Changes:

• Use #[proc_macro] at Rust 1.45+ to fix an issue where proc macros don't work with rust-analyzer (Use #[proc_macro] at Rust 1.45+ #2407)
• Support targets that do not have atomic CAS on stable Rust (Remove uses of unstable feature(cfg_target_has_atomic) #2400)
• futures-test: Add async #[test] function attribute (Async test function attribute #2409)
• Add stream::abortable (Abortable streams #2410)
• Add FuturesUnordered::clear (FuturesUnordered::clear #2415)
• Implement IntoIterator for FuturesUnordered (add FuturesUnordered::into_iter, make iter_pin_ref public #2423)
• Implement Send and Sync for FuturesUnordered iterators (impl Send + Sync for FuturesUnordered iterators #2416)
• Make FuturesUnordered::iter_pin_ref public (add FuturesUnordered::into_iter, make iter_pin_ref public #2423)
• Add SelectAll::clear (SelectAll::clear #2430)
• Add SelectAll::{iter, iter_mut} (expose iterators from SelectAll #2428)
• Implement IntoIterator for SelectAll (expose iterators from SelectAll #2428)
• Implement Clone for WeakShared (Add Clone to WeakShared #2396)