feat(transport): Add service multiplexing/routing (#99)

* feat(transport): Add service multiplexing/routing

This change introduces a new "router" built on top of
`transport::Server` that allows one to run multiple
gRPC services on the same socket.

```rust
Server::builder()
    .add_service(greeter)
    .add_service(echo)
    .serve(addr)
    .await?;
```

There is also a new `multiplex` example showcasing
server side service multiplexing and client side
service multiplexing.

BREAKING CHANGES: `Server::serve` is now crate private
and all services must be added via `Server::add_service`.
Codegen also returns just a `Service` now instead of a
`MakeService` pair.

Closes #29

Signed-off-by: Lucio Franco [email protected]

tonic-examples/helloworld-tutorial.md

@@ -191,7 +191,8 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
     let greeter = MyGreeter {};
 
     Server::builder()
-        .serve(addr, GreeterServer::new(greeter))
+        .add_service(GreeterServer::new(greeter))
+        .serve(addr)
         .await?;
 
     Ok(())
@@ -236,7 +237,8 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
     let greeter = MyGreeter {};
 
     Server::builder()
-        .serve(addr, GreeterServer::new(greeter))
+        .add_service(GreeterServer::new(greeter))
+        .serve(addr)
         .await?;
 
     Ok(())