A small, nonexhaustive set of warning fixes. The DiscIO Volume change
is a workaround for a GCC bug [1] that causes returning an unengaged
std::optional to emit annoying -Wmaybe-uninitialized warnings.
This last change alone fixes pages upon pages of warnings since
Volume.h is included from several files.
-Wstringop-truncation is another irrelevant warning for us, but
unfortunately there seems to be no way to disable it without
adding ugly pragmas wherever the warning appears.
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