Check if string is prefix of stralloc
#include <stralloc.h>
int stralloc_diffs(stralloc* a,const char* b);
stralloc_diffs returns negative, 0, or positive, depending on whether the \0-terminated string in a, without the terminating \0, is lexicographically smaller than, equal to, or greater than the string stored in a.
<0 if a<b, 0 if a==b, >0 if a>b.