Add Webassembly, Atomics, and other things

[mriise]

• Use atomics instead of spinlock (see https://matklad.github.io/2020/01/02/spinlocks-considered-harmful.html)
• Rolls up Web Assembly support from Add WebAssembly support. #3 (and make get_helper also return 64Ki), Web assembly does not support atomics, but since its a never changing value that is ok.
• Adds ability to test Web Assembly target
• Use feature gate std with default = ["std"] for more ergonomic use
• Bump rust edition to 2021 for cleaner looking modules & imports

Notes

This makes breaking changes.

Atomics not supported on some arm microcontrollers, In which case it will give a compiler error complaining about NonZeroUsize being missing, but that is another edge cage with an eventual solution with unstable #[cfg(target_has_atomic = ...)] and atomics polyfill
issue: rust-lang/rust/issues/32976