Make entity generation a new type and remove identifier

[ElliottjPierce]

Objective

This is a followup to #18704 . There's lots more followup work, but this is the minimum to unblock #18670, etc.

This direction has been given the green light by Alice here.

Solution

I could have split this over multiple PRs, but I figured skipping straight here would be easiest for everyone and would unblock things the quickest.

This removes the now no longer needed identifier module and makes Entity::generation go from NonZeroU32 to struct EntityGeneration(u32).

Testing

CI

[bushrat011899]

Mostly just some nitpicking. The changes look good and I love that this PR demonstrates the rolling updates we can do to migration guides.

[bushrat011899]

Future PR: might be nice to implement Iterator<Item = Self> for EntityGeneration. That gives an idiomatic next method.

[manokara]

Noticed some typos in the migration guide

[ElliottjPierce]

Benchmarks looks good, sparse set change detection was the only thing that was slowed down by anything notable. But I don't think that's a big deal. Other than that, I think this is generally an improvement in perf. (Less checks when changing the generation.)

Also want to point out that we loose a niche on EntityMeta here, but it can be added back shortly (via TableRow and ArchetypeRow). And I wanted to call out the big stability warning on Entity which gives me more confidence that this breaking change should be ok.