Support for Rc and Arc

[LtdJorge]

Hi Roman, will you add support for Rc and Arc? Were merged in rust-lang/rust#89132

[luketpeterson]

I started on this and did the initial pass adapting the Rc and Arc implementations from Rust 1.84.0. My work (wip) is here. https://github.com/luketpeterson/allocator-api2

I got bogged down because the Arc and Rc make heavy use of unstable memory layout APIs and allocator_api2 just imports core::alloc::Layout, etc.

It is certainly possible to avoid these APIs at the possible cost of a bit of performance - but this is where I got to today.

[luketpeterson]

As an aside, I'm also going to look at adapting triomphe to use allocator-api2 since triomphe is more like what I want anyway, and it might not be so deeply coupled to unstable (stdlib internal) memory layout features.

[luketpeterson]

Interestingly I'm not the only one running into this requirement for unstable features. https://www.youtube.com/watch?v=gr9v0FFXaZ8

This might be part of the solution. rust-lang/rust#123430