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.
Not only would the `(GAME != 2)` need to include TH03 in the future,
but this `#undef` trickery will blow up once we start reorganizing the
`#include`s in the next few commits.
Sadly, a few source files use master.lib's grcg_off() function in one
game and the macro in the other, which makes it infeasible to assign a
distinct name for every function.
Part of P0284, funded by [Anonymous] and Blue Bolt.
nmlgc