Feedback from porting Yasteroids

[kvark]

Speaking of kvark/yasteroids#2 - since it's the largest existing project using specs, I thought I'd share my impressions of using it at this scale:

• system interface is needed (System data problem #30), so that the systems are moved between threads, and their contents don't need to be syncable
• run shortcuts have a very limited use - only 2 out of 6 of the systems I got use it. We should focus instead on the custom run ergonomics, or empower the shortcuts with more abilities.
• overall it was mostly a pleasure to use it after simplecs, although checking for components in custom run functions is still quite noisy.
• it's non-trivial to get the BitSet performance by writing our custom runs
• dynamic entity creation/deletion works extremely nice and convenient. I used to do pooling of the like-entities in order to avoid creating them (which was simply not supported), but now the code is absolutely clean and lean.
• I had to fix a few places in specs in order for it to work. We need to establish strong rules about the order we lock things in (to avoid deadlocks).

[kvark]

Everything is fixed by #45 and #41, and #39