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.c b/src/set.c
index dcb3334..2249494 100644
--- a/src/set.c
+++ b/src/set.c
@@ -75,7 +75,7 @@
     free(set);
 }
 
-API uint8_t
+API ly_bool
 ly_set_contains(const struct ly_set *set, void *object, uint32_t *index_p)
 {
     LY_CHECK_ARG_RET(NULL, set, 0);