MINOR: standard: Add my_memspn and my_memcspn

Do the same than strsnp() and strcspn() but on a raw bytes buffer.
diff --git a/include/common/standard.h b/include/common/standard.h
index a6c3910..40991fa 100644
--- a/include/common/standard.h
+++ b/include/common/standard.h
@@ -1006,6 +1006,16 @@
  */
 const void *my_memmem(const void *, size_t, const void *, size_t);
 
+/* get length of the initial segment consiting entirely of bytes within a given
+ * mask
+ */
+size_t my_memspn(const void *, size_t, const void *, size_t);
+
+/* get length of the initial segment consiting entirely of bytes not within a
+ * given mask
+ */
+size_t my_memcspn(const void *, size_t, const void *, size_t);
+
 /* This function returns the first unused key greater than or equal to <key> in
  * ID tree <root>. Zero is returned if no place is found.
  */