Add signals with atomic usize

[thejpster]

An alternative signal implementation using AtomicUsize.

[gmmyung]

The recommended approach for libraries is to leave it up to the end user whether or not to enable this feature. (However, it may make sense to enable this feature by default for libraries specific to a platform where other implementations are known not to work.)

The portable-atomic crate advises to leave out cas implementation for unsupported tagets.

[thejpster]

If we left it to the end user binary to enable the cs feature on portable atomic we'd need a binary in the tree to do a test build. Or we could add a flag for test build purposes I guess. Or we could just not build for those targets.

[gmmyung]

What about adding a critical-section feature? The --all-features will make it available during test builds.

[thejpster]

What about adding a critical-section feature? The --all-features will make it available during test builds.

Good idea! Done. Have also serialised the tests.

[eldruin]

Sorry, could you add an entry to the changelog about this?

[thejpster]

Adding this to today's todo pile.