Remove rename of core to std in nostd builds

[AngelicosPhosphoros]

Motivation:

• It is confusing
• It makes work on my other PR (Rewrite set_logger function to use std::sync::Once on std #605) harder
• It makes harder to split nostd and std parts of crate

[Thomasdezeeuw]

I'm not going to accept this. We've had this convention for a long time and it works quite nicely once your past the initial surprise that std could also mean core. Furthermore we don't really want you use anything outside of core because that means that we'll break support for Rust targets without an OS.

[AngelicosPhosphoros]

Furthermore we don't really want you use anything outside of core because that means that we'll break support for Rust targets without an OS.

You yourself told me in #605 to use std primitive (std::sync::Once and std::sync::OnceLock). They available only in std, not in core.

Also, in my opinion, not accepting std code is easier when it is clearly visible what comes from core, and what comes from std.

[Thomasdezeeuw]

You yourself told me in #605 to use std primitive (std::sync::Once and std::sync::OnceLock). They available only in std, not in core.

However I also stated (#605 (comment)):

However, I don't know the reason why the current approach was taken over using Once. It could be that the lack of support for targets without an OS was a deal breaker, I'm not sure.

So I think we should use it, but if it's not support on all targets the crate supports we can't (or we need some work around as we have for AtomicUsize).