Replace lazy_static with once_cell

[jplatte]

Many other projects have switched from lazy_static to once_cell because it offers a macro-free solution to the same problem (with the only caveat being missing no_std support which I'm pretty sure is irrelevant for clap). Would you be open to a PR that does the same thing here?

In the future, the once_cell dependency can likely be removed in favor of the same functionality in std.

[matklad]

with the only caveat being missing no_std support

FWIW, the subset of the API which can be reasonably implented in no-std is available:

https://docs.rs/once_cell/latest/once_cell/race/index.html

The "I can't notify OS that I am blocked, so I am just going to spin" part is indeed not available, but I'd argue that is a feature :-)

[epage]

Huh, I thought there was a reason why I had kept it but not seeing a reason to. #3829 will switch us over.

[jplatte]

The "I can't notify OS that I am blocked, so I am just going to spin" part is indeed not available, but I'd argue that is a feature :-)

Hmm, I think that's what tracing wants.. (tokio-rs/tracing#1165 (comment)) 😕

I guess that also means matklad/once_cell#61 should be closed? In any case that issue is probably the place to continue this conversation.