switch to 2018 edition module structure

src/async_impl/mod.rs

@@ -1,12 +0,0 @@
-pub use self::body::{Body, Chunk};
-pub use self::decoder::{Decoder, ReadableChunks};
-pub use self::client::{Client, ClientBuilder};
-pub use self::request::{Request, RequestBuilder};
-pub use self::response::{Response, ResponseBuilderExt};
-
-pub mod body;
-pub mod client;
-pub mod decoder;
-pub mod multipart;
-pub(crate) mod request;
-mod response;