Add snapshot capability for event repository

[matthewadams]

NB: I'm just creating an issue to track what's been discussed in the below-referenced discussion.

Please add aggregate snapshot capability to event repository, as discussed at #164 (comment).

[idugalic]

An initial draft of EventSourcingSnapshottingAggregate and its variants is in place.

These interfaces are composed out of EventComputation/EventSnapshottingRepository and StateComputation/StateRepository interfaces, respectfully.

The application-vanilla module implements the handle method by composing these behaviors.

TODO:

• Implement application-arrow EventSourcingSnapshottingAggregate
• Test everything
• Adjust ref guide