Recursive AST walker that expands macros while preserving structural forms.

The walker traverses the AST top-down, applying these rules:

1. Structural forms (defmodule, def/defp, defmacro/defmacrop): preserved as-is, with their bodies recursively expanded.
2. Directives (alias, import, require): applied to the environment for subsequent expansions, preserved in output.
3. use: expanded by calling MACRO-__using__/2 directly, then re-walked.
4. Blocks (__block__): each statement walked sequentially, threading the env.
5. Expressions: expanded via :elixir_expand.expand/3 (or Macro.expand/2 fallback).
6. Failures: unexpandable nodes are kept with an @unexpanded marker prepended.