Implement/complete basic HTTP server.

There's already some basic functionality implemented: Phoenix code creates a listening socket and accepts new connections. Clients (e.g. browsers) can establish TCP connections and send HTTP requests to the Swift server code. The server code then responds with an HTTP response.

Ideally, we should implement a layer similar to JEE's Servlets or Python's Web Server Gateway Interface. This layer can then be used by frameworks or apps. We should also deliver a framework layer (similar to express on NodeJS or JAX-RS for Java) that plugs into the generic web service layer.

As a reference, see the battle-tested Servlet API: https://docs.oracle.com/javaee/7/api/javax/servlet/package-summary.html