Add `no_std` implementation based on `critical-section`.

[reitermarkus]

Add implementation based on critical-section for embedded targets.

This started out using critical_section::Mutex, but tests were failing since critical_section::with blocks all threads during initialization, so I changed it to be basically the same as imp_pl without the parking_lot parts, since atomic-polyfill is based on critical-section.

Depends on https://github.com/rust-embedded/critical-section/pull/26.

[reitermarkus]

Okay, so I have a different implementation which doesn't spin and simply completely blocks all threads during initialize, but this causes cargo test to deadlock unless using --test-threads 1.

[reitermarkus]

@matklad, can you have another look?

[reitermarkus]

@matklad, can you have another look at this? Thanks.

[matklad]

lgtm!

bors r+

[bors[bot]]

Build succeeded:

• Rust