Clever idea to use join!

This is great, I could not reason about how to effectively use fold here, building the number up by shifting the previous value makes a ton of sense.

the problem is the piecemeal allocations, maybe the runtime will make it not terrible,
but im sure not relying on that for hot code.

however i agree with how clean it is, i need to reach that level of iterator wizardry.
and if you could pre allocate the String vec then is not a issue
but then in this specific case simple concatenation in a big string would end being simpler.