This beefy commit removes the old (unnamed) Servant API.

This big commit ties the knot with the named _routes_ by providing
concrete server (i.e. handlers) implementations.

This big commit adds a separate module hierarchy for Servant named
routes (see https://www.tweag.io/blog/2022-02-24-named-routes/ ) which
will make working with servant endpoints more pleasant (especially when
it comes to emitted errors).

This still doesn't do anything to wire the routes to the concrete
handlers.