Expose recursive walk functionality via walk!

[directionless]

Fixes #267

This exposes the private treewalker! via a walk! interface. I opted to create a new walk! instead of making treewalk! public to avoid changing the signature on treewalker -- I could have made the tree argument have a default, but 🤷 I went this way. Happy to change it.

I added a note to the README, though I'm a bit on the fence. In favor, documenting this seems good for the next person. But, I think my table rename example might be too trivial. In my own testing (see #267) I needed something that feels much larger than belongs there. If it's a common enough need, it could be pulled into the library directly, but that feels like a deeper feature to consider.

In contrast, this is a pretty small patch.

Anyhow, that's a long winded explanation.

[lfittl]

Thanks for the contribution, and the README update - I think this looks great, and very helpful to make full use of the deparsing!