Use mbox instead of malloc_buf

[mehcode]

https://github.com/kennytm/mbox seems to have more 👀 on it then malloc_buf and given that this is a pretty niche area I think its worth it to switch. mbox also seems to have a nice extensive test suite too.

[SSheldon]

I wasn't aware of mbox, thanks for bringing it up @mehcode! It does look nice, but since this is a breaking change so I'd like to hold off on merging it until the next major version of objc is being published.

Unfortunately I don't actually have a timeline for the next major version... I'd love for that version to just be 1.0 and batch together all possible breaking changes. objc is apparently directly depended on by 38 different crates now, so I want to minimize any fragmenting of the ecosystem.

Let me know if you think this is more urgent. On my end, it doesn't seem like mbox is very widely used (crates.io reports 0 crates depending on it), and while it does have lots of neat functionality, the objc crate doesn't need most of that.

[mehcode]

My primary reason is mbox has an extensive test suite. Otherwise, no, there is no strong reasoning.