[FEATURE REQUEST] Make ECSSystem an interface

[lucasstarsz]

Is your feature request related to a problem? Please describe.

The main issue with Slope is that its current API is not yet simple enough. Based on the people who have heard of it up to this point, systems are still very confusing to players. Considering it currently hides things in plain sight (like the entities set and the world instance, as well as the more confusing concepts of how they are created and set up, I don't blame people for being confused.

What should the solution be like?

• Make ECSSystem an interface - streamline the process of receiving entities as well as standardizing how systems run.
• Add a standard run method that all ECSSystems have -- that way, this can be easily called by a third-party (such as a game engine.)