extensions CHANGE base for substatements support in extension instances

Proof of concept on Metadata annotation implementation. So far just some
of the structures, function stubs and the process framework. functions to
convert extension's substatements to lysp structures are still missing.
diff --git a/src/tree_schema_internal.h b/src/tree_schema_internal.h
index 1ff883d..209b8b2 100644
--- a/src/tree_schema_internal.h
+++ b/src/tree_schema_internal.h
@@ -650,7 +650,7 @@
 uint8_t lysc_iff_getop(uint8_t *list, int pos);
 
 /**
- * @brief Internal wrapper around lys_compile_extension() to be able to prepare list of compiled extension definition
+ * @brief Internal wrapper around lys_compile_extension() to be able to prepare list of compiled extension definitions
  * even for the parsed (not-implemented) module - see lys_module::off_extensions.
  *
  * @param[in] ctx_sc Compile context - alternative to the combination of @p ctx and @p module.
@@ -844,6 +844,6 @@
  *
  * return yang_keyword values.
  */
-enum yang_keyword lysp_match_kw(struct lys_parser_ctx *ctx, const char **data);
+enum ly_stmt lysp_match_kw(struct lys_parser_ctx *ctx, const char **data);
 
 #endif /* LY_TREE_SCHEMA_INTERNAL_H_ */