Consolidate duplicate implementations of compare_integers

Both Inertia and Twiddle previously included static implementations of this
exact same function, which was passed to `qsort()` as a comparator. With
this change, a single global implementation is provided in misc.c, which
will hopefully reduce code duplication going forward.

I'm refactoring this in preparation for the upcoming fallback polygon fill
function, which I'm about to add.

devel.but

@@ -5337,6 +5337,16 @@ equivalent to a bitwise-AND with \cw{~MOD_MASK}.
 Swap two regions of memory of \cw{size} bytes. The two regions must
 not overlap.
 
+\S{utils-compare-integers} \cw{compare_integers()}
+
+\c int compare_integers(const void *av, const void *bv);
+
+Compare the ints pointed to by \c{av} and \c{bv}. It returns an
+integer less than, equal to, or greater than zero if the first
+argument is respectively less than, equal to, or greater than the
+second. This function is intended to be passed to \c{qsort()} for
+sorting ints in ascending order.
+
 \C{writing} How to write a new puzzle
 
 This chapter gives a guide to how to actually write a new puzzle: