Assets::get_many

[Wuketuke]

fixes #16244

Assets<A> should have get_many / get_many_mut functions just like Query<...> does.
thats why I took the implementation of get_many_mut for Query as an inspiration.

i had to add an #[allow(unsafe_code)] above get_many_mut, so i want to know if thats permitted at that location

this pr also makes #16334 obsolete

Testing

I added two unit tests for get_many_mut (since get_many is trivial), but ill happily write more if there are some edge cases i overlooked (unsafe code is scarry)

[alice-i-cecile]

I think we should do this; Contentious is just for the use of unsafe (which I also think we should use here!).

[Wuketuke]

i dont understand why the ci is failing now
i just merged 1 change from main into my pr, and havent touched the places where these errors get thrown now
it passed just fine before the merge

[andriyDev]

This is broken at head currently https://discord.com/channels/691052431525675048/692572690833473578/1381857479520161803

I believe we're still trying to work out a fix.

[andriyDev]

Related issue: #19573

[Affinator]

What is the status of this? I could really use this for updating multiple assets in parallel...
How about adding it to the next milestone?

[andriyDev]

@Affinator with a little luck, we'll get #22939 in 0.20, and we'll get get_many for free through Query's implementation. I'll make sure to add that to the list of follow-up work for that!