Add feature gate for atomic load/store, gate spsc behind loadstore (Alternative fix for #271)

[Kriskras99]

The spsc module should only be build when atomic_polyfill is available. It's a big dependency (pulls in 12 extra dependencies) so for projects that don't need the atomics it's better to hide it behind a (default) feature gate.

I added two new features:

• loadstore that pulls in atomic_polyfill for architectures that need it and gates spsc
• atomics that enables loadstone and cas (replaces cas as the default feature)

Doing it this way should allow for more granular configuration of architectures, i.e. only pull-in atomic_polyfill if an architecture uses loadstore and doesn't have native support. This pull request does not contain that granularity as I'm not familiar with other architectures.

[TDHolmes]

But this will now unnecessarily pull in atomic-polyfill even for targets with native atomic support

[Kriskras99]

It only gets pulled in for targets which have 'atomic_polufill' as an (optional) dependency. For targets that support load store natively you can use cfgs in the Cargo.toml to only use 'atomic_polyfill' for 'cas'

[Kriskras99]

It only gets pulled in for targets which have 'atomic_polufill' as an (optional) dependency. For targets that support load store natively you can use cfgs in the Cargo.toml to only use 'atomic_polyfill' for 'cas'

This is slightly wrong. It's true that specifying atomic_polyfill as a feature only pulls it in as an optional dependency. However it's not possible to only pull it in for cas for some architectures. For that we'd need something like this.