lang: replace stack-based parser with a recursive parser

lang/ast/do.c

@@ -0,0 +1,16 @@
+#include "iterate.h"
+#include "node.h"
+
+#include <fx/ds/string.h>
+
+static void collect_children(
+	struct ivy_ast_node *node, struct ivy_ast_node_iterator *iterator)
+{
+	struct ivy_ast_do_node *do_node = (struct ivy_ast_do_node *)node;
+	ast_node_iterator_enqueue_node(iterator, node, do_node->n_block);
+}
+
+struct ast_node_type do_node_ops = {
+	.n_collect_children = collect_children,
+	.n_node_size = sizeof(struct ivy_ast_op_node),
+};