Investigate Objective C support

[JoshuaKGoldberg]

After discussing with @JonesMB, there are two main concerns:

1. There are no native namespaces. Everything is global, so although you can directly import files, you can't have any naming conflicts.
2. It's standard (and, in some cases, necessary) to split files into .h and .m in C/C++ form. Lovely.

1 is doable, and likely necessary for PHP.
2 will require an Objective C-specific secondary compiler step. It'll need to go through every generated class (.m) and generate the corresponding .h.