Fix duplicated chilren in Scene spawn

[lassade]

Fixes #889,

TLDR;

The Problem

1. A entity is been added twice in Children component while spawning a scene
2. Performance takes a massive hit, transform_propagate_system took 13ms in release with a small skeleton tree ~20 bones
3. Affects gltf loading
4. Right now the only "correct" way of creating a Scene is using WorldBuilder.with(Parent(...)) (still wrong for other reasons)

The Fix

1. Impl FromResources and MapEntities for PrevousParent
2. Checking for duplicates in the Children using a debug_assert
3. Children.0 and PreviousParent.0 are now pub(crate), their contents are a reflect of Parent

Conclusion
This is the best short term fix, it will make so that the only true correct way of creating Scene will be WorldBuilder.with_children which has no caviats

[lassade]

@Moxinilian the CI error is very strange can you force a re-run to see if fixes?

[Moxinilian]

This is an issue with all CI currently, I don't have permission to do anything about it. I'll make sure cart is aware.

[cart]

I think this is a reasonable short term fix. I made one small change (see comment below)

[cart]

we need some way to re-order children. order already matters for things like UI, and once we have an editor it will matter for "scene organization".

I added this to enable those scenarios:

children.swap(a_index, b_index);