reduce system boilerplate

[jmmk]

• Which frames to run on (every 3, every 5, etc.)
• Which entities to work on (based on components)
• Function that returns a list of entities to associate (currently a custom list comprehension inside each system)

[jmmk]

Might look something like this:

(defsystem attack [world]
  {:entities [attacking [:attacker :attacking]
              alive [:attackable :alive]]
   :frame-count 15
   :fn do-attacks})