Thoughts on zipped tree traversal

I wanted to pick up from our discussion in https://github.com/FluxML/Functors.jl/pull/32, in particular around your comments in https://github.com/FluxML/Functors.jl/pull/32#issuecomment-1026454917. I think one concrete example to work off of is a function like `torch.nn.Module.load_state_dict` [^1]. In other words, something that prefers but does not require similarly structured trees (it returns a list of discrepancies if they aren't).

I feel like this is absolutely something StructWalk could excel at, but I agree with your point that creating a one-size-fits-all mechanism would be difficult. My question would be whether there might be some extensible mechanism to handle this, just like how `WalkStyle` allows for highly customizable traversal behaviour over individual nodes.

[^1]: this came up in the discussion around restoring saved model weights, ref. @darsnack's post at https://github.com/FluxML/Flux.jl/issues/1027#issuecomment-1034226504.