Clean-up libflake headers

[fzakaria]

Motivation

I ran include-what-you-use over libflake as it seemed small to start.

steps:

> iwyu_tool.py -p build/ src/libflake-tests/ -j 100 -- -Xiwyu --mapping_file=/nix/store/0zqzzg1mm1fman18svxkf5p4km4yq85p-include-what-you-use-0.24/share/include-what-you-use/boost-all.imp -Xiwyu --cxx17ns > iwyu.out

> cd build

> fix_includes.py < ../iwyu.out

I then went through the header files and now I can more safely remove the header files clangd warns that are "not used".

Context

There is an ongoing debate on whether to do include-what-you-use (iwyu).

For:

• https://github.com/include-what-you-use/include-what-you-use/blob/master/docs/WhyIWYU.md

Against:

• https://danra.prose.sh/dont_use_iwyu

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