Upgrade to Tokio 1.x, futures 0.3, Hyper 0.14, etc (#459)

* Upgrade dependencies, oneshot channel ref

* New service style?

* Fix warning

* A server is running again

* Working server with async blocks

* UI working again

* Finish upgrade

* Bump MSRV to 1.46.0 for if/match in const fn

* Update the README as part of this

src/cli/build.rs

@@ -50,7 +50,7 @@ impl BuildCommand {
         write_model(&session, &self.output, output_kind)?;
 
         if self.watch {
-            let mut rt = Runtime::new().unwrap();
+            let rt = Runtime::new().unwrap();
 
             loop {
                 let receiver = session.message_queue().subscribe(cursor);