Provide `entities_iter()` on `EntityMapper` to get an iterator of foreign entities

[aristaeus]

What problem does this solve or what need does it fill?

I want to get a list of all the entities that an EntityMapper is tracking. If we have a concrete hash map this is easy, but in generic code we have to work with the trait.

What solution would you like?

Add a new function to EntityMapper (returning Vec rather than impl Iterator for trait object reasons)

fn entities(&self) -> Vec<(Entity, Entity)>;

What alternative(s) have you considered?

Add a separate trait with this function. This would reduce breakage by not requiring the ecosystem to immediately implement a new function.