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.h b/src/tree_schema.h
index 4a3dbec..eeb0fd7 100644
--- a/src/tree_schema.h
+++ b/src/tree_schema.h
@@ -162,6 +162,89 @@
 #define LYS_AUGMENT 0x2000
 
 /**
+ * @brief List of YANG statements
+ */
+enum ly_stmt {
+    LY_STMT_NONE = 0,
+    LY_STMT_ACTION,
+    LY_STMT_ANYDATA,
+    LY_STMT_ANYXML,
+    LY_STMT_ARGUMENT,
+    LY_STMT_AUGMENT,
+    LY_STMT_BASE,
+    LY_STMT_BELONGS_TO,
+    LY_STMT_BIT,
+    LY_STMT_CASE,
+    LY_STMT_CHOICE,
+    LY_STMT_CONFIG,
+    LY_STMT_CONTACT,
+    LY_STMT_CONTAINER,
+    LY_STMT_DEFAULT,
+    LY_STMT_DESCRIPTION,
+    LY_STMT_DEVIATE,
+    LY_STMT_DEVIATION,
+    LY_STMT_ENUM,
+    LY_STMT_ERROR_APP_TAG,
+    LY_STMT_ERROR_MESSAGE,
+    LY_STMT_EXTENSION,
+    LY_STMT_FEATURE,
+    LY_STMT_FRACTION_DIGITS,
+    LY_STMT_GROUPING,
+    LY_STMT_IDENTITY,
+    LY_STMT_IF_FEATURE,
+    LY_STMT_IMPORT,
+    LY_STMT_INCLUDE,
+    LY_STMT_INPUT,
+    LY_STMT_KEY,
+    LY_STMT_LEAF,
+    LY_STMT_LEAF_LIST,
+    LY_STMT_LENGTH,
+    LY_STMT_LIST,
+    LY_STMT_MANDATORY,
+    LY_STMT_MAX_ELEMENTS,
+    LY_STMT_MIN_ELEMENTS,
+    LY_STMT_MODIFIER,
+    LY_STMT_MODULE,
+    LY_STMT_MUST,
+    LY_STMT_NAMESPACE,
+    LY_STMT_NOTIFICATION,
+    LY_STMT_ORDERED_BY,
+    LY_STMT_ORGANIZATION,
+    LY_STMT_OUTPUT,
+    LY_STMT_PATH,
+    LY_STMT_PATTERN,
+    LY_STMT_POSITION,
+    LY_STMT_PREFIX,
+    LY_STMT_PRESENCE,
+    LY_STMT_RANGE,
+    LY_STMT_REFERENCE,
+    LY_STMT_REFINE,
+    LY_STMT_REQUIRE_INSTANCE,
+    LY_STMT_REVISION,
+    LY_STMT_REVISION_DATE,
+    LY_STMT_RPC,
+    LY_STMT_STATUS,
+    LY_STMT_SUBMODULE,
+    LY_STMT_TYPE,
+    LY_STMT_TYPEDEF,
+    LY_STMT_UNIQUE,
+    LY_STMT_UNITS,
+    LY_STMT_USES,
+    LY_STMT_VALUE,
+    LY_STMT_WHEN,
+    LY_STMT_YANG_VERSION,
+    LY_STMT_YIN_ELEMENT,
+    LY_STMT_EXTENSION_INSTANCE,
+
+    LY_STMT_SYNTAX_SEMICOLON,
+    LY_STMT_SYNTAX_LEFT_BRACE,
+    LY_STMT_SYNTAX_RIGHT_BRACE,
+
+    LY_STMT_ARG_TEXT,
+    LY_STMT_ARG_VALUE
+};
+
+/**
  * @brief Extension instance structure parent enumeration
  */
 typedef enum {