Reorder headers to follow the Google C++ Style Guide:
> In dir/foo.cc or dir/foo_test.cc:
>
> 1. dir2/foo2.h.
> 2. A blank line
> 3. C system headers (more precisely: headers in angle brackets with
the .h extension), e.g., <unistd.h>, <stdlib.h>.
> 4. A blank line
> 5. C++ standard library headers (without file extension), e.g.,
<algorithm>, <cstddef>.
> 6. A blank line
> 7. Other libraries' .h files.
> 8. A blank line
> 9. Your project's .h files.
https://google.github.io/styleguide/cppguide.html#Names_and_Order_of_Includes
This feeds into efforts to create a working install target.
The order of headers is still funky, and was "fixed" by clang-format,
but in a way that doesn't exactly align with the style guide. Further
cleanup of header order is coming in a follow-up PR.
Joey Parrish
Cosmin Stejerean
Jacob Trimble
KongQun Yang