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/Cargo.toml

@@ -52,6 +52,14 @@ path = "src/tls_client_auth/server.rs"
 name = "tls-client-auth-client"
 path = "src/tls_client_auth/client.rs"
 
+[[bin]]
+name = "multiplex-server"
+path = "src/multiplex/server.rs"
+
+[[bin]]
+name = "multiplex-client"
+path = "src/multiplex/client.rs"
+
 [[bin]]
 name = "gcp-client"
 path = "src/gcp/client.rs"