Add server operation registry and router (#850)

This commit adds two things:

1. A runtime router implementing `tower`'s
   [`Service`](https://docs.rs/tower-service/0.3.1/tower_service/trait.Service.html)
   that adheres to [Smithy's `http` trait
   specification](https://awslabs.github.io/smithy/1.0/spec/core/http-traits.html#http-trait),
   that is linear in the number of registered routes.
2. A code-generated "operation registry" that allows service
   implementers to provide Rust functions and declare them as the
   handlers for their service's operations.

The framework will receive HTTP requests from the server and route them
to the corresponding operation handler.