Pull changes from bevy_hecs

[multun]

The history is still very messy and will get plenty of rework: commits will get squashed, splitted, until the result looks good

EDIT: Blocked by #71, we need to decide what needs to get in before splitting this

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[multun]

@googlebot I signed it!

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[Ralith]

Closing as no longer applicable, especially as bevy has diverged. Thanks for your interest, and feel free to open new PRs if there are specific changes you think would still make sense for us!