A component for web apps that need access to services (stateful
components.) It mainly functions as a container for all the
components that the web app handlers need. It provides a middleware
to inject those components into ring requests.

To use, initialize by calling `app` with a config map (unused for
now) and a handler function. In your component system map, add any
other components that your app needs. Call `app-handler` with the
app component to return a handler wrapped in a middleware that will
inject all the components into the request map, under a
`::services` key, keyed by the same keys specified in the system map.

Handlers can use the `service` helper function to fetch a
particular service out of a request map.

Ring request/response related specs.

Immutant web server component.