libyang FEATURE introduce and use new ly_bool type

To indicate simple flags or true/false return values, use a standalone
ly_bool type.

We do not use stdbool's bool type to avoid need to mimic all its
features on platforms that do not provide it. ly_bool is just a simple
rename for uint8_t and the reason to use it is only a better readability
of the meaning of the variables or function's return values.
diff --git a/src/set.h b/src/set.h
index eebc7b3..1a2269d 100644
--- a/src/set.h
+++ b/src/set.h
@@ -121,9 +121,9 @@
  * @param[in] set Set to explore.
  * @param[in] object Object to be found in the set.
  * @param[out] index_p Optional pointer to return index of the searched @p object.
- * @return Boolean value (0 is false) if the @p object was found in the @p set.
+ * @return Boolean value whether the @p object was found in the @p set.
  */
-uint8_t ly_set_contains(const struct ly_set *set, void *object, uint32_t *index_p);
+ly_bool ly_set_contains(const struct ly_set *set, void *object, uint32_t *index_p);
 
 /**
  * @brief Remove all objects from the set, but keep the set container for further use.