TraverseAncestry::current_address to get address of current node

[overlookmotel]

See: oxc-project/oxc#6881 (comment)

We want to avoid implementing GetAddress for &T because it's error-prone. But often in transformer you only have a &T not a &Box<T> and want to get it's address (e.g. in a visitor like exit_function).

This is legitimate because the &mut T passed into visitor function is always a reference to value in the arena.

We could achieve this by:

• Pushing Ancestor to the stack of ancestors before calling enter_* and pop it after calling exit_*.
• TraverseAncestry::parent return not stack.last() but 1 ancestor before it.
• TraverseAncestry::ancestor return the item at index last_index - level - 1 (not last_index - level).
• TraverseAncestry::ancestors skip the last item on stack.
• Always push to ancestors stack in all walk_* functions (I think we skip that for leaf nodes at present).

Then existing APIs behave as they do now, but TraverseAncestry::current_address can get the address of the current node with stack.last().address().

Problem: What about visitors for enums e.g. Expression? current_address would return the address of parent node, because enums don't get an Ancestor. Either:

1. Live with it. or
2. Move pushing to ancestor stack up into the walk_* functions for enums (a bit tricky, because some nodes don't always live within an enum).