Add `Timer.Multi.Engine` and `Timer.Multi.Actor`.

[jemc]

As an MVP for this library, we created Timer.Engine and Timer.Actor, which together enable the basic use case of a single timer per actor.

However, we eventually want to also support the use case of a single actor efficiently supporting multiple timers, the ability to add and remove timers from the set, etc.

We likely want to use a similar approach to the one used in Pony: a set of hierarchical timing wheels. However, we'll still be different in that we allow the user to define the actor implementation and just embed the engine inside it - as opposed to in Pony where the user-defined notifier object(s) are embedded inside the library-defined actor.

Also, we will continue to support the single-timer use case as a totally separate use case (via Timer.Engine and Timer.Actor) because it is a much simpler, more efficient, and more convenient way to approach that use case than forcing everything into the multi-timer use case as Pony does.