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.
This will end up at the end of Mima's translation unit. Might as well
start a separate temporary one then, which will simplify the diff once
we eventually merge it into that future Mima TU which is very likely to
grow from top to bottom.
Part of P0279, funded by Yanga and Blue Bolt.
Adding op/, main/, and end/ directories does nicely cover a great
majority of the "not really further classifiable slices" implied in
d56bd45.
Part of P0086, funded by [Anonymous] and Blue Bolt.
nmlgc