Add portable-atomic feature and disable portable-atomic/critical-section by default

[taiki-e]

Closes #264

This implements the first of the ways I mentioned in #264 (comment).

• New Cargo feature to use portable-atomic's atomic types instead of core's

(The second way mentioned there was probably more user-friendly, but I did not choose it because it would have caused portable-atomic to appear in Cargo.lock as a platform-specific dependency. Like cfg-specific dependencies in crossbeam-rs/crossbeam#487 (comment).)

No new test case was added, because the following case is sufficient to test the situation where the race module is enabled (due to default features) and portable-atomic is used instead of core::sync::atomic (due to critical-section feature enables portable-atomic feature).

once_cell/xtask/src/main.rs

Line 40 in 72f7c2e

cmd!(sh, "cargo test --features critical-section").run()?;

This also closes #248 since this is a sound way to do that.

cc @kaspar030 @brodycj

[matklad]

This all looks alright to me, thanks a lot for putting this together!

[taiki-e]

Hmm, due to Cargo bug (rust-lang/cargo#10801), this makes portable-atomic to appear in Cargo.lock (even if the portable-atomic feature is not enabled. even if portable-atomic will never be compiled).

With the diagnostic improvements in portable-atomic 1.8 (taiki-e/portable-atomic#181), the need for require-cas feature has decreased, so I think it is probably fine to remove "portable-atomic?/require-cas"...

[matklad]

I am also fine with keeping it! Cargo bug is a bug, the code here seems correct, and could perhaps be helpful for older versions of portable-atomic?

I guess you are the best person to make a judgement call here though :)

[brodycj]

Thanks @taiki-e & @matklad & thanks for the quick release!