Adds these missing functions

Set.difference
Set.intersection
Set.union
Set.symmetricDifference
Set.isSubsetOf
Set.isSupersetOf
Set.isDisjointFrom

Also adds convinience function Set.toArray which lets you get the array of the set directly

Before (still valid):

arr->Set.fromArray->Set.values->Array.fromIterator

New, shorter:

arr->Set.fromArray->Set.toArray

Creating a set from an array and then get the array of the set is a common pattern in javascript to remove duplicates from an array

ie

  let set = Set.fromArray(["apple", "orange", "apple", "banana"])
  set->Set.toArray // ["apple", "orange", "banana"]

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Set