Upstream stdx changes

[BenjaminBrienen]

I have some changes on my version of stdx that I figured I might as well PR back to r-a.

Let me know if there are things that you want me to undo.
Almost all of the changes come from following clippy lints which are not currently enabled in this workspace.

[Veykril]

Why all the random inlines? And most of the must use also seems unnecessary to me (aside from the constructors/builders).

[BenjaminBrienen]

I just followed a lint to add the inlines and must_use. must_use is recommended whenever the function doesn't appear to mutate its arguments, so it doesn't make sense to call the function if you don't need the resulting value.

[Veykril]

I find putting those on a bunch of functions blindly to be more noise than helpful. We don't run clippy with all pedantic lints precisely because a lot of those lints are subjectively questionable.

As for inline specifically, none of that code is perf critical so we can just rely on the compiler doing the right thing (what it considers right). In fact, a lint telling you about when to use inline makes no sense to me. If the lint knows, the compiler should know either way. So I'd at least like to see the inline attributes removed here. must_use I don't care too much about (as those are reasonable here either way).

[BenjaminBrienen]

Let me know if there are any other changes you want me to undo.

[Veykril]

Thanks!