Consider renaming "System" to something more intuitive.

[rj00a]

Systems in evenio behave differently from systems in other ECS frameworks. This makes discussions involving other libraries more confusing than it needs to be. We should consider renaming them.

Potential Candidates:

• Handler, as in "event handler"
• Listener, as in "event listener"
• Observer, as in the observer pattern. This is what flecs calls them.

Handler is my first pick. "event listener" seems to be less common of a name, and "observer" has too many object-oriented associations for my taste.