runtime: rename current_thread -> basic_scheduler (#1769)

It no longer supports executing !Send futures. The use case for
It is wanting a “light” runtime. There will be “local” task execution
using a different strategy coming later.

This patch also renames `thread_pool` -> `threaded_scheduler`, but
only in public APIs for now.

tokio/tests/rt_basic.rs

@@ -0,0 +1,34 @@
+#![warn(rust_2018_idioms)]
+
+use tokio::runtime::Runtime;
+use tokio::sync::oneshot;
+use tokio_test::{assert_err, assert_ok};
+
+use std::thread;
+use std::time::Duration;
+
+#[test]
+fn spawned_task_does_not_progress_without_block_on() {
+    let (tx, mut rx) = oneshot::channel();
+
+    let mut rt = rt();
+
+    rt.spawn(async move {
+        assert_ok!(tx.send("hello"));
+    });
+
+    thread::sleep(Duration::from_millis(50));
+
+    assert_err!(rx.try_recv());
+
+    let out = rt.block_on(async { assert_ok!(rx.await) });
+
+    assert_eq!(out, "hello");
+}
+
+fn rt() -> Runtime {
+    tokio::runtime::Builder::new()
+        .basic_scheduler()
+        .build()
+        .unwrap()
+}