Merge async-rs#208

208: add stream::join r=stjepang a=yoshuawuyts Ref async-rs#129 async-rs#187. This adds the `stream::join` which can join multiple streams into a single stream. Thanks! ## Example ```rust let a = stream::once(1u8); let b = stream::once(2u8); let c = stream::once(3u8); let mut s = stream::join!(a, b, c); assert_eq!(s.collect().await, vec![1u8, 2u8, 3u8]); ``` ## Screenshot ![Screenshot_2019-09-17 async_std stream - Rust](https://user-images.githubusercontent.com/2467194/65080099-cedb8b00-d9a0-11e9-984f-edd7d6bad5cc.png) Co-authored-by: Yoshua Wuyts <yoshuawuyts@gmail.com>

Author: bors[bot]

Cargo.toml

@@ -25,6 +25,7 @@ docs = []
 unstable = []
 
 [dependencies]
+async-macros = "1.0.0"
 async-task = "1.0.0"
 cfg-if = "0.1.9"
 crossbeam-channel = "0.3.9"

src/stream/mod.rs

@@ -42,5 +42,9 @@ cfg_if! {
  pub use double_ended_stream::DoubleEndedStream;
  pub use from_stream::FromStream;
  pub use into_stream::IntoStream;
+
+ #[cfg_attr(feature = "docs", doc(cfg(unstable)))]
+ #[doc(inline)]
+ pub use async_macros::{join_stream as join, JoinStream as Join};
  }
 }