feat(client): expose hyper::client::connect::Connect trait alias

This is *just* a "trait alias". It is automatically implemented for all
`Service<Uri>`s that have the required bounds. It's purpose being public
is to ease setting trait bounds outside of hyper. Therefore, it doesn't
have any exposed associated types, to prevent otherwise relying on any
super-traits that hyper requires.

src/client/mod.rs

@@ -150,8 +150,6 @@ impl Client<(), Body> {
 impl<C, B> Client<C, B>
 where
     C: Connect + Clone + Send + Sync + 'static,
-    C::Transport: Unpin + Send + 'static,
-    C::Future: Unpin + Send + 'static,
     B: Payload + Send + 'static,
     B::Data: Send,
 {
@@ -548,8 +546,6 @@ where
 impl<C, B> tower_service::Service<Request<B>> for Client<C, B>
 where
     C: Connect + Clone + Send + Sync + 'static,
-    C::Transport: Unpin + Send + 'static,
-    C::Future: Unpin + Send + 'static,
     B: Payload + Send + 'static,
     B::Data: Send,
 {