feat(client): Make client an optional feature

cc #2223

BREAKING CHANGE: The HTTP client of hyper is now an optional feature. To
  enable the client, add `features = ["client"]` to the dependency in
  your `Cargo.toml`.

src/proto/h2/client.rs

@@ -10,7 +10,7 @@ use tokio::io::{AsyncRead, AsyncWrite};
 
 use super::{decode_content_length, ping, PipeToSendStream, SendBuf};
 use crate::body::HttpBody;
-use crate::common::{task, Exec, Future, Never, Pin, Poll};
+use crate::common::{task, exec::Exec, Future, Never, Pin, Poll};
 use crate::headers;
 use crate::proto::Dispatched;
 use crate::{Body, Request, Response};