Return types for `ecs_find!` queries

[recatek]

For ecs_find! and ecs_find_borrow! queries, we could support having the query macro parse a return type and return it (while also bundling it with the bool for whether or not the entity was found):

let (found, result) = ecs_find!(world, entity, |a: &CompA| -> u64 { /* ... */ })

Where found is a bool and result is the u64 we requested to return.

[recatek]

Done in 0.3.0