Merge Services and Modules.

The distinction was very confusing, and the use of 'module' (which means
something specific in Ruby) to mean something different was unnecessarily
confusing.

Incidentally, the Server Launcher is now called a Service.

Breaking changes to config
==========================

* The `modules` directive is now `services`;
* Any import from `service_common`/`ServiceCommon` is now `services`/`Services`;
* The server service is now called `Rapid::Server::Service`, at
  `ury_rapid/server/service`.