Only recompute space-view heuristics if something has changed

[jleibs]

Specifically, entities_per_system_per_class (and the things it depends on) can be trivially cached if we have not inserted any new components in the data-store.

An easy cache-key for this is to simply sum the number of components across all entity-paths.

[jleibs]

This is very related to the new store view functionality.

[Wumpf]

Related (almost duplicate to):

• Re-engineer the space-view heuristics engine #4388