OK, this is the big one. We still keep using `#include` guards only
where we absolutely need to, but with each header now being valid in
isolation, this can now actually help *minimize* the length of each
translation unit's `#include` list. Turns out that after removing all
the duplicates, we only *actually* need to guard 29 headers across all
5 games.
Part of P0285, funded by [Anonymous] and iruleatgames.
In most cases, this discovery will mean that we have to bite the bullet
and remove `const` from pointer parameters if the generated ASM
wouldn't match otherwise. That by itself doesn't really simplify the
code, but at least we get to remove a single bloated `reinterpret_cast`
from one function call already.
Part of P0278, funded by Yanga.
nmlgc