Rework how Knobs are handled

[Daniel Díaz Carrete]

Knobs are controls associated to some components which allow controlling the component from an HTTP endpoint, kind of like what the "actuator" framework does for Spring Boot.

Right now, if we want to add a component with a Knob, we have to register it in two places: in the main composition root, and in the Servant "runner" component.

This is annoying, it would be better to specify it in only one place. A further annoyance is that we need to add two coomponents to the composition root: the component-with-knob and the bare component.

[Daniel Díaz Carrete]

The following solution is implemented in c9ce51a:

• An alternative to the Constructor type has been added called AccumConstructor. This latter type embodies the notion of constructors that, besides reading from the final environment and producing a record-of-functions of a specific type, also read and produce a monoidal accumulator value.
• In our case, we use a map of Knob values as the accumulator.
• The logger component produces a KnobMap value containing a single entry, the Knob for the logger.
• The KnobServer component reads the accumulated KnobMap and produces a Servant server for querying and setting all knobs (although currently there's only the logger Knob.)

With these changes, we don't need to update in two places whenever we add a component accompanied by a Knob. It's enough to update the composition root!