Adding 64-bit support, fallback for unsupported platforms, and more

[DouglasDwyer]

Hey! While working on a separate project, I found this crate. It looked great, but I needed support for 64-bit futexes in my project. So, I created this fork. While working on the fork, I got a bit carried away and added:

• Support for AtomicI32, AtomicI64, and AtomicU64.
• A fallback futex implementation using a table of condvars for platforms that don't have native support (like Linux)
• Native support for wasm32 (nightly only)
• Support for waiting with timeout

I wanted to reach out and see whether you were interested in merging any of these into this crate. It would help prevent ecosystem fracture, and expand the capability of a crate that's already widely-used. Still, these changes are somewhat opinionated (for example, I know it was stated on another issue that you liked the simplicity of atomic-wait without a manual table fallback) and I made some breaking changes. So I'm also happy to leave my work as a fork if you feel it beyond the scope of this crate. Cheers!