Add a replacement for `std::sync::Arc` that uses `portable-atomic::AtomicUsize`

[notgull]

There should be an alloc feature that provides a replacement for Arc that uses portable_atomic::AtomicUsize instead of the standard library version. This would allow users of portable_atomic to use Arc on targets where atomics are not supported but allocation is.

However, this may be better suited for an alternate crate instead of this crate. I can implement either if desired.

[taiki-e]

Sounds good.
I was thinking it would be nice to have a crate (something like portable-atomic-util or portable-atomic-extra) to put in utilities like #8 that are more difficult to decide about API than existing APIs, but putting Arc in it seems like a good idea.