feature: iterator chain / concatenation

[keturn]

Python calls it itertools.chain:

chain("ABC", [1, 2, 3]).toArray() === ["A", "B", "C", 1, 2, 3]

Like Array.concat, but lazy, and doesn't need its iterables to be marked with Symbol.isConcatSpreadable.

(does the toolkit not have an Iterator Utilities section yet?)