Explicit execution order ambiguities API

[Ratysz]

This API allows marking systems as exempt from ambiguity detection with other systems with the same mark. E.g., this will not report any ambiguities:

SystemStage::parallel()
    .with_system(conflicts_with_everything.system().label("0").before("1").before("2"))
    .with_system(conflicts_with_everything.system().label("1").in_ambiguity_set("a"))
    .with_system(conflicts_with_everything.system().label("2").in_ambiguity_set("a"))
    .with_system(conflicts_with_everything.system().label("3").after("1").after("2"))

It's a stopgap solution until some form of #1375 is implemented.

[alice-i-cecile]

I like this: it will help us prepare for 0.5 by cleaning up the default system ambiguities, and we'll still need this tool once we have archetype invariants.

Are you planning to integrate this with #1423 or just leave these as stringly-typed until we get #1375? I can see either option being defensible.

[Ratysz]

Are you planning to integrate this with #1423 or just leave these as stringly-typed until we get #1375?

Yes, definitely - it's easy enough to warrant doing this even for a "temporary" solution. This could be left out of #1423, for expediency, I'll just do a follow-up PR when it lands. Or it lands before this and I'll rebase 🤷‍♂️

[cart]

I think this is good to go (once we resolve the one open thread)

[Ratysz]

Ah, dang, rustfmt snuck in an unrelated change. It should be fine, just annoying.