Implement WalkTransforming traversal

[willscott]

• there's probably a better algorithm for the skip/don't skip subset of interests that are to be recursively traversed versus what's set up here.
• there might be a way to combine the list and map recursion methods
• should think about a reasonable set of test cases / fixtures

[warpfork]

+1 from me. (Not a detailed review; but it looks about right.)

I'm... probably not going to harp on test fixtures for this too much, for once (surprising though that might be). I think this function may be a golang-specific cul-de-sac a bit, and therefore I care a lot less.

[willscott]

I considered if this needs to add the partial skip logic added in #358. I believe in the same way that it is not used for WalkLocal it isn't directly needed for WalkTransforming (or can be added later if it becomes needed)