Add support for precompilation

[cocoa-xu]

This PR adds support for precompilation. (linking PR at elixir-nx/stb_image#22).

I'll add more inline docs etc, but the code should be mostly okay for a first review. :)

[cocoa-xu]

Hi @josevalim, this should be ready for review now. :)

I've added a precompilation guide for creating a precompiler module and using a precompiler. A demo precompiler module is available on cocoa-xu/cc_precompiler, and an example project using this demo precompiler is available on cocoa-xu/cc_precompiler_example.

I tried to make the precompilation document as easy to read and follow as possible, but please feel free to rephrase or change the wording in any part of that precompilation document directly. If there is anything I didn't make clear, please let me know, and I'll explain and have another try on that part.

[josevalim]

Thank you @cocoa-xu! I will merge this PR and do any improvements locally. I will ping back here once it is ready to go!

[josevalim]

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