Using iwyu

[noamzaks]

The following tool may be useful:

iwyu

[calcmogul]

I tried this in https://github.com/calcmogul/allwpilib/tree/include-what-you-use a month ago, and I rebased it today. I had to make manual fixes afterward to most files because it would:

1. Include system internal headers that throw errors if you include them directly.
2. Include headers that don't exist on some platforms.
3. Include internal Eigen headers that don't compile unless other headers are included first. (I guess Eigen doesn't include what it uses. 🙂)
4. Put forward declarations of the same type in both the header and source, then fail to actually include the header containing the type in the source file.

I'm still not exactly happy with the contents of that branch yet.

[calcmogul]

We replaced all of our external native dependencies with PyPI packages in #242 to make wpiformat installation and usage easier. Since iwyu doesn't have a similar package, we can't depend on iwyu.