portable-atomic-util: Rewrite Arc based on std::sync::Arc

[taiki-e]

When I reviewed #41, I thought the differences in its implementation were fine, but in reality, we have missed several problems that would not have occurred if the implementation had been based on std::sync::Arc (#137, #139, ¹).

This will not necessarily be easy, since the standard library uses many unstable features.

Footnotes

1. UPDATE: There are also cases where there are implementations that do not exist in the std::sync::Arc... ↩

[notgull]

When I was originally writing the Arc code, I tried to follow along with the standard library's version, but polyfilling bits and pieces in when unstable features were used.

It might be nice to bump the MSRV of portable-atomic-util higher than 1.34 so we gain access to some of these features, such as addr_of.

[taiki-e]

The implementation is in #142. addr_of_mut in as_ptr is not really important; the layout calculation is needed anyway to implement from_raw, and it can be used in as_ptr, so making the layout calculation work in the older version is the important point.

[taiki-e]

As a side effect of this work, I found potential UBs in std Arc/Rc.
rust-lang/rust#119241