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.

Cargo.toml

@@ -60,7 +60,7 @@ default = [
 ]
 runtime = [
     "tcp",
-    "tokio/time",
+    "tokio/rt-core",
 ]
 tcp = [
     "net2",