support for writing car files about apple-platform-rs

[indygreg]

https://blog.timac.org/2018/1018-reverse-engineering-the-car-file-format/ seems like a useful reference. The author produced code at https://github.com/Timac/CARParser/.

I then did a search for various magic values in that post. I stumbled across https://github.com/facebookarchive/xcbuild/tree/dbaee552d2f13640773eb1ad3c79c0d2aca7229c/Libraries/libcar and https://github.com/Reflejo/pycar, and https://github.com/iineva/bom/blob/b998a3aac1c0e863cfc92fdfca74adf8931b831e/pkg/asset/decode.go as other implementations.

Haven't looked at them at all and don't know if they are comprehensive enough to do what we need.

I do know Facebook invested heavily in Linux based tooling for Apple platform development. It would not surprise me if Facebook was able to do application building + packaging purely from Linux. So I suspect that this archived Facebook repo is a trove of information that can bypass a lot of reversing work.