Add a reference counted container

[notgull]

This pull requests resolves #37 by adding Arc and Weak types to the portable-atomic-util crate that use portable_atomic::AtomicUsize in their operation. The goal is to allow for portable using of Arc on platforms that may have allocation but not atomics. Arduino AVR comes to mind for this.

These new types are based off of the ones in libstd, but I explicitly avoid using unstable features. I do not implement unstable functions, like new_uninit. I also avoided certain functions like new_cyclic and take_mut because I didn't get around to it.

[notgull]

It looks like, since embedded targets don't come with an allocator, building portable-atomic-util fails on those targets. I've just removed --workspace from build.sh for now

[taiki-e]

Thanks! This looks great to me.

[notgull]

Hmm, it's failing on nightly when building with bpfeb-unknown-none with errors like these:

 error[E0599]: no method named `fetch_add` found for reference `&portable_atomic::AtomicUsize` in the current scope
   --> portable-atomic-util/src/arc.rs:152:46
    |
152 |         let old_size = self.inner().strong().fetch_add(1, Relaxed);
    |            

Looks like, on BPF targets, the compare_exchange-type functions are unavailable.

[taiki-e]

Ah, it's #29.

Could you add args+=(--exclude portable-atomic-util) to the following line for now?

portable-atomic/tools/build.sh

Line 221 in a304903

bpf*) ;; # TODO

[notgull]

Thanks!

[taiki-e]

Thanks again!

[taiki-e]

bors r+

[bors]

Build succeeded:

• ci
• test (aarch64-apple-darwin)
• test (aarch64-unknown-linux-gnu)
• valgrind (aarch64-unknown-linux-gnu)