Implement new custom ECS (WIP)

[caelunshun]

Status

• [ ] Ready
• [x] Development
• [ ] Hold

Description

Implements #402. Work in progress.

Progress

• [x] Spawn and despawn entities
• [x] Random component access
• [x] Static and dynamic queries
• [x] Handle Drop components
• [x] Dynamic borrow checking and mutable component access
• [x] Pass all hecs tests with Miri
• [x] Move system executor to new ECS
• [x] Switch to the new ECS in Feather (should be trivial; the API is almost the same as hecs)
• [ ] Integrate with quill

I'm going to hold off on the new event handling system and the bundles discussed in #402 for another PR.

[caelunshun]

I updated the PR with a checklist of remaining work.

[Defman]

BUMP

[Defman]

Bump