SystemDictionary: add a cache for allBehaviors

[MarcusDenker]

I think if we cache allBehaviors instead of classNames, everything would be much faster.

(first benchmark shows that then it is faster to use "#allBehaviors do:" instead of "allBehaviorsDo:", which means in turn we can then use advanced collection operations (e.g. flatCollect:) to simplify things a lot (and speed up even more)