entity responsibility moved to engine

deleted entity states, changed examples
components are now values instead of pointers
internal entity collection is no longer exported
specialized entity lists and event subscription
system as event subscriber, engine as broker

completed remodelling, untested!
working examples
reorganizing message pub/sub engine
component responsibility moved halfway to engine

still bugged with nil entities and components!
shifted component responsibility completely
cleanup
unified observer lists

had to disable message priority waiting due to lock problems
reuse entity ids
entity and aspect conversion benchmarks
limit message sending goroutines

sorting directly after subscribe to prevent lock collision
added missing error checks to asteroid example
Merge remote-tracking branch 'origin/feature/ecs' into feature/concurrent-ecs
adjusted systems and entity getter/setter
Merge remote-tracking branch 'origin/feature/concurrent-ecs' into feature/ecs