add stralloc_diff and stralloc_diffs

stralloc/stralloc_diff.c

@@ -0,0 +1,14 @@
+#include "stralloc.h"
+#include "byte.h"
+#include "str.h"
+
+extern int stralloc_diff(const stralloc* a,const stralloc* b) {
+  register unsigned int i;
+  register int j;
+  for (i=0;;++i) {
+    if (i==a->len) return i==b->len?0:-1; if (i==b->len) return 1;
+    if ((j=(a->s[i]-b->s[i]))) return j;
+  }
+  return j;
+}
+