Provide Stack.fromList?

I'm looking to use this library but I need to encode & decode my data structure to json. I would find it useful if there was a `Stack.fromList` to match the `Stack.toList` so I could base encoders & decoders on those. I guess this library could also provide generic encoders & decoders but I realise that pulling in `elm/json` could be considered inappropriate for such a core data structure.

