data tree CHANGE move macros to functions to cleanup API

Remove lyd_node_children() since it is just an old duplication of
LYD_CHILD and LYD_CHILD_NO_KEYS macros. As a consequence, these macros
are converted to the functions together with LYD_PARENT (same names, but
lowercase) since they have similar functionality in the API.
diff --git a/src/diff.c b/src/diff.c
index 3e3dfbc..0c28bf4 100644
--- a/src/diff.c
+++ b/src/diff.c
@@ -98,7 +98,7 @@
         diff_parent = match;
 
         /* move down in the diff */
-        siblings = LYD_CHILD_NO_KEYS(match);
+        siblings = lyd_child_no_keys(match);
     }
 
     /* duplicate the subtree (and connect to the diff if possible) */
@@ -591,7 +591,7 @@
 
         /* check descendants, if any, recursively */
         if (match_second) {
-            LY_CHECK_GOTO(lyd_diff_siblings_r(LYD_CHILD_NO_KEYS(iter_first), LYD_CHILD_NO_KEYS(match_second), options,
+            LY_CHECK_GOTO(lyd_diff_siblings_r(lyd_child_no_keys(iter_first), lyd_child_no_keys(match_second), options,
                                               0, diff), cleanup);
         }
 
@@ -909,7 +909,7 @@
             }
         } else {
             /* none operation on nodes without children is redundant and hence forbidden */
-            if (!LYD_CHILD_NO_KEYS(diff_node)) {
+            if (!lyd_child_no_keys(diff_node)) {
                 LOGINT_RET(ctx);
             }
         }
@@ -971,7 +971,7 @@
     }
 
     /* apply diff recursively */
-    LY_LIST_FOR(LYD_CHILD_NO_KEYS(diff_node), diff_child) {
+    LY_LIST_FOR(lyd_child_no_keys(diff_node), diff_child) {
         LY_CHECK_RET(lyd_diff_apply_r(lyd_node_children_p(match), match, diff_child, diff_cb, cb_data));
     }
 
@@ -1238,7 +1238,7 @@
             diff_match->flags |= src_diff->flags & LYD_DEFAULT;
         } else {
             /* but the operation of its children should remain DELETE */
-            LY_LIST_FOR(LYD_CHILD_NO_KEYS(diff_match), child) {
+            LY_LIST_FOR(lyd_child_no_keys(diff_match), child) {
                 LY_CHECK_RET(lyd_diff_change_op(child, LYD_DIFF_OP_DELETE));
             }
         }
@@ -1278,7 +1278,7 @@
                                       diff_match->flags & LYD_DEFAULT ? "true" : "false", NULL));
         } else {
             /* keep operation for all descendants (for now) */
-            LY_LIST_FOR(LYD_CHILD_NO_KEYS(diff_match), child) {
+            LY_LIST_FOR(lyd_child_no_keys(diff_match), child) {
                 LY_CHECK_RET(lyd_diff_change_op(child, cur_op));
             }
         }
@@ -1292,8 +1292,8 @@
         LY_CHECK_RET(lyd_diff_change_op(diff_match, LYD_DIFF_OP_DELETE));
 
         /* all descendants not in the diff will be deleted and redundant in the diff, so remove them */
-        LY_LIST_FOR_SAFE(LYD_CHILD_NO_KEYS(diff_match), next, child) {
-            if (lyd_find_sibling_first(LYD_CHILD(src_diff), child, NULL) == LY_ENOTFOUND) {
+        LY_LIST_FOR_SAFE(lyd_child_no_keys(diff_match), next, child) {
+            if (lyd_find_sibling_first(lyd_child(src_diff), child, NULL) == LY_ENOTFOUND) {
                 lyd_free_tree(child);
             }
         }
@@ -1323,7 +1323,7 @@
 
     assert(diff);
 
-    child = LYD_CHILD_NO_KEYS(diff);
+    child = lyd_child_no_keys(diff);
     mod = ly_ctx_get_module_latest(LYD_CTX(diff), "yang");
     assert(mod);
 
@@ -1397,7 +1397,7 @@
     LY_CHECK_RET(lyd_diff_get_op(src_diff, &src_op));
 
     /* find an equal node in the current diff */
-    lyd_diff_find_node(diff_parent ? LYD_CHILD_NO_KEYS(diff_parent) : *diff, src_diff, &diff_node);
+    lyd_diff_find_node(diff_parent ? lyd_child_no_keys(diff_parent) : *diff, src_diff, &diff_node);
 
     if (diff_node) {
         /* get target (current) operation */
@@ -1434,7 +1434,7 @@
         diff_parent = diff_node;
 
         /* merge src_diff recursively */
-        LY_LIST_FOR(LYD_CHILD_NO_KEYS(src_diff), child) {
+        LY_LIST_FOR(lyd_child_no_keys(src_diff), child) {
             LY_CHECK_RET(lyd_diff_merge_r(child, diff_parent, diff_cb, cb_data, diff));
         }
     } else {