Fix TypeScript error in diffArray

There are four errors inside diffArray that causes builds to fail. One of them is:

Argument of type 'T[]' is not assignable to parameter of type 'ComparedItem[]'.
  Type 'T' is not assignable to type 'ComparedItem'. ts(2345)
index.ts(39, 27): This type parameter might need an `extends ComparedItem` constraint.

Adding extends ComparedItem (as in stringifyArray, objectifyArray, etc) removes the errors, and builds now work again.