Replace `objc` with `objc2`

[madsmtm]

Redo of https://github.com/servo/core-foundation-rs/pull/513, see that for previous discussion and motivation (GitHub Actions was giving me trouble).

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I've focused on making this PR as minimal as possible, that is, I'm explicitly not trying to actually use any of the new features that objc2 offers - we can discuss those separately as we get there. So as-is, this PR is only a small improvement on the status quo, though still definitely an improvement.

The main difference you have to know for now is that all arguments and return types must implement a specific marker trait objc2::encode::Encode, which makes it possible to catch type mistakes at runtime, when debug assertions are enabled (e.g. cases of passing an u32 where an isize was expected). This is a huge boon to soundness, and the few mistakes I've found and corrected are likely only the tip of the iceberg - I could use someone's help to run this on bigger code-bases that exercise more of cocoa/cocoa-foundation.

Feel free to ask any questions about anything!

CC recent contributors @jrmuizel, @mukilan, @jdm and @michaelwright235.