The upcoming `#include` cleanup is going to spell out all implicit
dependencies of every header file. This would
• `#include "master.hpp"` in every header that uses `PF_FN_LEN`, and
• `#include "pc98.h"` in `master.hpp`, adding lots of graphics
declarations to translation units that don't need them.
Moving `PF_FN_LEN` to its own file not only avoids needless inclusions
of `master.hpp` and all of its future dependencies later, but already
removes `master.hpp` from 6 translation units – not to mention that we
no longer need to preprocessor-guard some of the usages of `PF_FN_LEN`.
Part of P0284, funded by [Anonymous] and Blue Bolt.
nmlgc