executor: remove Executor & TypedExecutor traits (#1724)

The `Executor` trait is sub-optimal as it forces a `Box<dyn Future>` to
spawn. Instead, `tokio::spawn` delegates to the specific runtime
implementation set for the current execution context.

`TypedExecutor`, while useful, has seen limited adoption. As such, it is
removed from `tokio` proper. Moving it to `tokio-util` is a possibility
that can be explored as follow up work.

tokio/tests/executor.rs

@@ -1,24 +0,0 @@
-#![warn(rust_2018_idioms)]
-
-use tokio::executor::DefaultExecutor;
-
-use std::future::Future;
-use std::pin::Pin;
-
-mod out_of_executor_context {
-    use super::*;
-    use tokio::executor::Executor;
-
-    fn test<F, E>(spawn: F)
-    where
-        F: Fn(Pin<Box<dyn Future<Output = ()> + Send>>) -> Result<(), E>,
-    {
-        let res = spawn(Box::pin(async {}));
-        assert!(res.is_err());
-    }
-
-    #[test]
-    fn spawn() {
-        test(|f| DefaultExecutor::current().spawn(f));
-    }
-}

tokio/tests/executor_global.rs

@@ -1,17 +0,0 @@
-use tokio::executor::{with_default, DefaultExecutor};
-
-#[test]
-fn default_executor_is_send_and_sync() {
-    fn assert_send_sync<T: Send + Sync>() {}
-
-    assert_send_sync::<DefaultExecutor>();
-}
-
-#[test]
-#[should_panic]
-fn nested_default_executor_status() {
-    let _enter = tokio::executor::enter().unwrap();
-    let mut executor = DefaultExecutor::current();
-
-    let _result = with_default(&mut executor, || ());
-}