runtime: merge multi & single threaded runtimes (#1716)

Simplify Tokio's runtime construct by combining both Runtime variants
into a single type. The execution style can be controlled by a
configuration setting on `Builder`.

The implication of this change is that there is no longer any way to
spawn `!Send` futures. This, however, is a temporary limitation. A
different strategy will be employed for supporting `!Send` futures.

Included in this patch is a rework of `task::JoinHandle` to support
using this type from both the thread-pool and current-thread executors.

tokio/tests/process_issue_42.rs

@@ -3,7 +3,7 @@
 #![warn(rust_2018_idioms)]
 
 use tokio::process::Command;
-use tokio::runtime::current_thread;
+use tokio::runtime;
 
 use futures_util::future::FutureExt;
 use futures_util::stream::FuturesOrdered;
@@ -18,7 +18,8 @@ fn run_test() {
     let finished_clone = finished.clone();
 
     thread::spawn(move || {
-        let mut rt = current_thread::Runtime::new().expect("failed to get runtime");
+        let mut rt = runtime::Builder::new().current_thread().build().unwrap();
+
         let mut futures = FuturesOrdered::new();
         rt.block_on(async {
             for i in 0..2 {