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 .PTN functions, vector functions, and egc_copy_rect_1_to_0_1()
(finally!) from TH01, as well as playfld.hpp from all games(finally!),
together with a bunch of other functions in their vicinity.
Part of P0201, funded by Ember2528 and Yanga.
nmlgc