add helper method Query::with_entity

[Nathan-Fenner]

Objective

While I'm writing systems, I frequently find myself suddenly wanting to have access to the Entity that I'm iterating over with a Query<&MyComp> - so I have to refactor to Query<(Entity, &MyComp)>. This means changing the parameter type and other iteration sites over the same query, even if those sites don't use the Entity.

Solution

This PR adds a convenience method, .with_entity() analogous to e.g. .enumerate() on Iterator - it returns a QueryLens over the same data, but with Entity added to the data selector (in other words, it turns a Query<D, F> into a QueryLens<(Entity, D), F>).

Testing

• There's a trivial doctest that should make sure it compiles; this is essentially just a convenience wrapper around transmute_lens_filtered but without the need to specify generic types as a caller

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Changelog

• Added Query::with_entity() convenience function that returns a QueryLens over the same results but also including the Entity id

[alice-i-cecile]

I like this quite a bit, but I think we should change the name to transmute_something. That way it will be clearer in the auto-complete / methods for beginners, and will be grouped with its siblings. transmute_with_entity is fine, but I like transmute_include_entity a bit better since there's no With involved.

[Victoronz]

In 13.0, Entity is the only parameter that can be unconditionally added, but in 14.0, &Archetype, and EntityLocation work like this as well, and this list might expand in the future.
Maybe it makes more sense to have a generic transmute_include/transmute_add?

[hymm]

Why do you need to transmute to add an entity instead of just including it in the original query?

[Nathan-Fenner]

Maybe it makes more sense to have a generic transmute_include/transmute_add?

Maybe, but the main point of this particular method is to avoid needing to specify (lengthy) generic types and get better inference - so even if we had a generic method, I'd still want the Entity-specific version, since it's common (and the only one of those that I actually have wanted so far)