Header files are now self-contained, which makes the relationships
between the files clearer, and crucially allows LSP tools like `clangd`
to function correctly in all of our header files. I have verified that
the headers are self-contained (aside from the various Windows shims) by
compiling them as if they were C files – in a follow-up commit I plan to
add this to CI to ensure we don't regress on this front.
This was promised in the review of the introduction of geom_grow, but would have
been painful to do there because of the series that introduced it. Now that
those are comitted, renaming is easier.
