libyang REFACTOR avoid constant literals

Improve readability of the code.

Includes also unification of "no break in the case" comment to "fall
through".
diff --git a/src/set.c b/src/set.c
index 1eb229b..5ed43f4 100644
--- a/src/set.c
+++ b/src/set.c
@@ -141,10 +141,12 @@
     }
 
     if (set->size == set->count) {
-        new = realloc(set->objs, (set->size + 8) * sizeof *(set->objs));
+#define SET_SIZE_STEP 8
+        new = realloc(set->objs, (set->size + SET_SIZE_STEP) * sizeof *(set->objs));
         LY_CHECK_ERR_RET(!new, LOGMEM(NULL), LY_EMEM);
-        set->size += 8;
+        set->size += SET_SIZE_STEP;
         set->objs = new;
+#undef SET_SIZE_STEP
     }
 
     if (index_p) {