Use portable-atomic only for 32-bit architectures

[repi]

A new take on how to use the atomics without bringing in the portable-atomic crate for 64-bit architectures where one can use just std.

Believe this approach could satisfy the requirements, and be easier to use for everyone, but I'm not super familiar with this crate so may have missed something also.

Resolves #346
Resolves #323

Changed

• Only use portable_atomic::AtomicU64 for 32-bit platforms, for 64-bit platforms use std.

Added

• Re-export AtomicU64 so one have an easy and convenient way to get the used atomic counter type regardless if one 32-bit or 64-bit architectures and without depending on external crates when using this crate.

Removed

• Removed the std-atomics Cargo feature as it is not longer needed

[tobz]

I ended up replicating this by hand in a3a706b as I wanted to slightly tweak some things. I'm going to close this PR with that in mind, but I did make sure to credit you for that change in the CHANGELOG, as it ultimately overlapped with your diff by nearly 100%.

Thanks again for your contribution and kicking my butt to get this solved in a cleaner way.

[repi]

thanks!

[tobz]

Released as metrics@v0.21.0.

Thanks again for your contribution!