Use portable-atomic

[chrysn]

This should not have run-time impact on platforms that support atomic boolean compare-and-swap, but enables its use on microcontrollers where the user configures portable-atomic in such a way that atomics can be emulated (eg. through disabling interrupts).

I have not tested whether portable-atomics works with the 1.31 version tested in CI. Should that fail, should I rather feature-gate this, can we update the MSRV of this crate, or should I hack CI to use an older version of portable-atomics (because Rust 1.31 was way before cargo became rust-version aware)?

[chrysn]

As CI is only run once cleared, I've tested this locally with Rust 1.31. (And found that the minimal version was incorrect, that is now fixed).

Inconveniently, even the oldest usable portable-atomics version (1.3.0) does not work with Rust 1.31.

Is updating the MSRV of this project an option? I haven't bisected, but at least 1.58 works (and works even with portable-atomics 1.70); if we need to stay lower but can move from 1.31, I'm happy to provide a sharper result than "> 1.31 <= 1.58".