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/client/conn.rs

@@ -25,7 +25,7 @@ use tower_service::Service;
 
 use super::dispatch;
 use crate::body::HttpBody;
-use crate::common::{task, BoxSendFuture, Exec, Future, Pin, Poll};
+use crate::common::{task, exec::{BoxSendFuture, Exec}, Future, Pin, Poll};
 use crate::proto;
 use crate::rt::Executor;
 #[cfg(feature = "http1")]

src/client/mod.rs

@@ -62,7 +62,7 @@ use http::{Method, Request, Response, Uri, Version};
 use self::connect::{sealed::Connect, Alpn, Connected, Connection};
 use self::pool::{Key as PoolKey, Pool, Poolable, Pooled, Reservation};
 use crate::body::{Body, HttpBody};
-use crate::common::{lazy as hyper_lazy, task, BoxSendFuture, Future, Lazy, Pin, Poll};
+use crate::common::{lazy as hyper_lazy, task, exec::BoxSendFuture, Future, Lazy, Pin, Poll};
 use crate::rt::Executor;
 
 #[cfg(feature = "tcp")]

src/client/pool.rs

@@ -11,7 +11,7 @@ use futures_channel::oneshot;
 use tokio::time::{Duration, Instant, Interval};
 
 use super::Ver;
-use crate::common::{task, Exec, Future, Pin, Poll, Unpin};
+use crate::common::{task, exec::Exec, Future, Pin, Poll, Unpin};
 
 // FIXME: allow() required due to `impl Trait` leaking types to this lint
 #[allow(missing_debug_implementations)]
@@ -777,7 +777,7 @@ mod tests {
     use std::time::Duration;
 
     use super::{Connecting, Key, Pool, Poolable, Reservation, WeakOpt};
-    use crate::common::{task, Exec, Future, Pin};
+    use crate::common::{task, exec::Exec, Future, Pin};
 
     /// Test unique reservations.
     #[derive(Debug, PartialEq, Eq)]