Extract UI nodes into a Vec

[ickshonpe]

Objective

Extract UI nodes into a Vec instead of an EntityHashMap.

Solution

Extract UI nodes into a Vec instead of an EntityHashMap.
Store an index into the Vec in each transparent UI item.
Compare both the index and render entity in prepare so there aren't any collisions.

Showcase

Yellow this PR, Red main

cargo run --example many_buttons --release --features trace_tracy

extract_uinode_background_colors

extract_uinode_images

prepare_uinodes

[alice-i-cecile]

How does an EntityIndexSet compare? @Victoronz may have ideas.

[Victoronz]

It seems that the speedup gained from just not hashing at all outweighs the slowdown from the n^2 filtering required to retrieve it again.
While EntityIndexMap could be faster than the EntityHashMap case, I think the Vec here would still be faster than both.
What I am however curious about is this:
Given that it was a HashMap before, there was no defined order, which means that if we are storing these items in a Vec instead, we could use the ordering to signify where an item is stored.

Instead of using indexes, you could sort the initial list of keys for which you want to push stuff by Entity. When retrieving, you just sort the keys the same way, and you can retrieve the items in the same manner, no filtering required.
That is what I would guess to be fastest here!
(This only works if you push and retrieve everything at once each time)

[alice-i-cecile]

Leaving Victoronz suggestions for follow-up; they're great but this is a clear huge win.