Settle on #include style

[rr-]

tests/actions_tests.cc vs src/main.cc

[fmang]

Here's my preferred order for header inclusion, roughly based on relevance:

• headers defining the interface we implement (if applicable),
• headers part of our project,
• library headers,
• system headers.

This order helps ensure headers include their own dependencies.

I usually separate these groups with a blank line, and sort them using the default alphetical-ish order.

I'll leave this issue open to discussion for a week, then will fix the existing files. It's hardly an issue in my opinion though, but I don't wanna fall into the “style doesn't matter” category.