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.
By leaving out just the one `score_delta = 0;` assignment, ZUN created
a function with semantics so complex that it already required naming
all other score-related functions just to make sense of it.
Part of P0242, funded by Yanga.
nmlgc