`transform()` for in-place mapping

[Lea Verou]

We discussed this somewhere but never created an issue.

Transforming an AST in place can also be useful. I propose a transform() function for that.
map() could then be defined in terms of transform(), by using a callback that always returns a new object.

Note that transforming in place poses a few challenges to avoid infinite loops.

[Adam Janicki]

I wrote a transform function, and I'm still working on rewriting map to call transform; figuring out the copying is proving to be more difficult than I expected