lang: ast: convert RPN expression queue to ast and replace generic EXPR node with it

lang/ast/unit.c

@@ -25,6 +25,13 @@ static struct token_parse_result parse_class_keyword(
 	return PARSE_RESULT(IVY_OK, 0);
 }
 
+static struct token_parse_result parse_expr_begin(
+	struct ivy_parser *ctx, struct ivy_token *tok)
+{
+	parser_push_state(ctx, IVY_AST_EXPR);
+	return PARSE_RESULT(IVY_OK, PARSE_REPEAT_TOKEN);
+}
+
 static enum ivy_status add_child(
 	struct ivy_ast_node *parent, struct ivy_ast_node *child)
 {
@@ -55,4 +62,7 @@ struct ast_node_type unit_node_ops = {
 		KW_PARSER(CLASS, parse_class_keyword),
 		KW_PARSER(USE, parse_use_keyword),
 	},
+	.n_expr_parser = {
+		.expr_begin = parse_expr_begin,
+	},
 };