Compare two strallocs
#include <stralloc.h>
int stralloc_diff(const stralloc* a,const stralloc* b);
stralloc_diff returns negative, 0, or positive, depending on whether a is lexicographically smaller than, equal to, or greater than the string b.
<0 if a<b, 0 if a==b, >0 if a>b.