Sync-Point ergonomics

[minecrawler]

Sync-Points should be

• [x] easy to create and extend (with other sync-points) 50c66d90ee6a2745adfe727dc1b521f62351993d 2839b08567906f1d34c1bd944095d7ba12143dfc
• [x] able to have labels, which allow arbitrary insertion of systems before or after them af4e9bb3a6c30bbd6dc12093883ecabc683e2d49
• [x] serializable as prefabs, so a complex pipeline can be created separately, stored and loaded and have a very simple syntax 50c66d90ee6a2745adfe727dc1b521f62351993d
• [x] safe to use - we need checks for loops! bceb42150ce4bcec2a0ebdf9e82a0e99c5652522
• [x] execute stuff on point (commands.flush(), world.maintain(), custom logic like "extract to other thread") bb7ecae41a84ae90f42bd50f0bfbd313e378e0ef

[minecrawler]

upon investigation, it is not really possible to make a good API to insert systems around sync-point because systems are part of stages. As such, that part will not be implemented and it remains with the user to create good pipelines (which is best practice anyway)