feat(rt): introduce rt::Executor trait

The `hyper::rt::Executor` trait allows defining custom executors to be
used with hyper's `Client` and `Server`.

Closes #1944

BREAKING CHANGE: Any type passed to the `executor` builder methods must
  now implement `hyper::rt::Executor`.

  `hyper::rt::spawn` usage should be replaced with `tokio::task::spawn`.

  `hyper::rt::run` usage should be replaced with `#[tokio::main]` or
  managing a `tokio::runtime::Runtime` manually.

src/rt.rs

@@ -5,4 +5,4 @@
 //! If the `runtime` feature is disabled, the types in this module can be used
 //! to plug in other runtimes.
 
-//pub use crate::common::Executor;
+pub use crate::common::Executor;