Add an API module

An API lets another program read and drive a running Apranik; see APIs. The module value implements spec.APIModule (settings, Info, NewAPIServer()), and the server it builds implements spec.APIServer:

• Start(ctx, coreProvider) binds its own listener, returns once it is listening, and keeps serving in goroutines it owns; a taken port is Start's error;
• Close() stops serving, ends every open request and waits;
• everything it knows of the running core comes from spec.APICoreProvider, whose methods are small and separate; an API imports spec and nothing from core;
• a method that cannot answer returns a spec.ErrAPI... sentinel; map it with errors.Is.

Test the module with a fake APICoreProvider and no core. Register it with moduleRegistry.APIs().Register(<name>.NewAPIModule).