feat(client): change Connect trait into an alias for Service

The `Connect` trait is now essentially an alias for
`Service<Destination>`, with a blanket implementation as such, and is
sealed.

Closes #1902

BREAKING CHANGE: Any manual implementations of `Connect` must instead
  implement `tower::Service<Destination>`.

src/service/mod.rs

@@ -38,10 +38,13 @@ pub use tower_service::Service;
 
 mod http;
 mod make;
+mod oneshot;
 mod util;
 
 pub(crate) use self::make::{MakeConnection, MakeServiceRef};
 pub(crate) use self::http::HttpService;
+pub(crate) use self::oneshot::{oneshot, Oneshot};
 
 pub use self::make::make_service_fn;
 pub use self::util::service_fn;
+