extension instances CHANGE access to the extension instance's substatements

Provide libyang a transparent access to the substatements of a specific
extension instances for various generic processing.

The patch introduces lysc_ext_substmt() to simplify work with the newly
added lysc_ext_instance.substmts sized array providing the extension
instance's substatements.
diff --git a/src/tree_schema.h b/src/tree_schema.h
index 7e25fb7..546c587 100644
--- a/src/tree_schema.h
+++ b/src/tree_schema.h
@@ -1395,6 +1395,8 @@
     LYEXT_SUBSTMT insubstmt;         /**< value identifying placement of the extension instance */
     LYEXT_PARENT parent_type;        /**< type of the parent structure */
     struct lysc_ext_instance *exts;  /**< list of the extension instances ([sized array](@ref sizedarrays)) */
+
+    struct lysc_ext_substmt *substmts; /**< list of allowed substatements with the storage to access the present substatements */
     void *data;                      /**< private plugins's data, not used by libyang */
 };