PathValue should be usable in dict and set

[ovangle]

When dealing with trees, it's quite common to use a dictionary to label nodes, using the path as the key. Currently this can be accomplished by casting the path to str, but this introduces a lot of unnecessary casts.

Ideally, PathValue should extend tuple instead of list, but since this would be a breaking change, perhaps it's better to implement __hash__ on PathValue?

I'd imagine most code would treat paths as immutable anyway.

Thoughts?

I'd be happy to submit a PR implementing whatever you decide is the best course of action.