Add joinMap

[ursi]

I was using List.map with String.join until I realized that that goes through the whole list twice. I tried List.foldl but then I realized that couldn't get me the same behaviour as String.join, where the separator is only inserted in between strings.

[ursi]

Thinking about it more, I guess you can just fold if you check to see if the accumulation is the empty string and handle it differently (I might change the implementation to that since it seems like it would be faster). That being said, having experienced using joinMap in my own code, I still would prefer it be its own function.

[ursi]

Apparently recursion is faster https://ellie-app.com/8rJw9pbcTpBa1