The translation units were probably a better place back when most of
the codebase was still compiled in C mode, we only had a few C++ TUs,
almost everything needed to be declared as `extern "C"`, and moving
these declarations into the headers would have been really noisy with
all the `#ifdef __cplusplus` / `#endif` required. Nowadays though,
we've greatly reduced that surface area. And given that headers will
include even more headers as part of the upcoming `#include` cleanup,
it makes sense to make the jump now.
Part of P0284, funded by [Anonymous] and Blue Bolt.
The 2022 edition of "fundamental KAJA function I haven't looked into
more deeply during all these years"…
Part of P0225, funded by Yanga, Blue Bolt, and 32th System.
The TH04 one might have the same function structure, but the only thing
that's actually identical in both games is the picture darkening loop.
Part of P0119, funded by [Anonymous] and -Tom-.