Universal binaries for macOS

[robbert-vdh]

Creating this issue as a reminder for myself to implement this. Cargo does not support universal binaries out of the box (rust-lang/cargo#8875) which is why I haven't added this yet, but adding them should not be as I initially thought. This would involve a new build flag that builds for both the native target and for the 'other' target (either x86_64-apple-darwin or aarch64-apple-darwin), and then replaces the binary in target/{debug,release} with a lipo'd version before continuing. I've done something similar in clap-validator here: https://github.com/robbert-vdh/clap-validator/blob/6386b97cc8eafc7c9439550ddf27acaa1700bfe5/.github/workflows/build.yml#L128