schema CHANGE always store extension's parent statement

So far the parent statement of extension instances was stored only in
case the extension was not stored directly in the extension list of
parent structure. But there is no reason to distinguish these cases, so
the patch makes libyang to store the extension's parent statement type
always.
diff --git a/src/tree_schema_helpers.c b/src/tree_schema_helpers.c
index 717f2b9..ab5b0d0 100644
--- a/src/tree_schema_helpers.c
+++ b/src/tree_schema_helpers.c
@@ -1073,6 +1073,43 @@
     }
 }
 
+API enum ly_stmt
+lys_nodetype2stmt(uint16_t nodetype)
+{
+    switch (nodetype) {
+    case LYS_CONTAINER:
+        return LY_STMT_CONTAINER;
+    case LYS_CHOICE:
+        return LY_STMT_CHOICE;
+    case LYS_LEAF:
+        return LY_STMT_LEAF;
+    case LYS_LEAFLIST:
+        return LY_STMT_LEAF_LIST;
+    case LYS_LIST:
+        return LY_STMT_LIST;
+    case LYS_ANYXML:
+        return LY_STMT_ANYXML;
+    case LYS_ANYDATA:
+        return LY_STMT_ANYDATA;
+    case LYS_CASE:
+        return LY_STMT_CASE;
+    case LYS_RPC:
+        return LY_STMT_RPC;
+    case LYS_ACTION:
+        return LY_STMT_ACTION;
+    case LYS_NOTIF:
+        return LY_STMT_NOTIFICATION;
+    case LYS_USES:
+        return LY_STMT_USES;
+    case LYS_INPUT:
+        return LY_STMT_INPUT;
+    case LYS_OUTPUT:
+        return LY_STMT_OUTPUT;
+    default:
+        return LY_STMT_NONE;
+    }
+}
+
 const char *
 lys_datatype2str(LY_DATA_TYPE basetype)
 {