Iterator objects gain a join() method

Iterator.prototype.join lands in the spec (6db9135)

ECMA-262 now defines Iterator.prototype.join(separator), which consumes an iterator, stringifies its values, and concatenates them with a separator (defaulting to ,).

It also spells out iterator-safe abrupt completion handling and skips undefined/null values during concatenation, making the new API's behavior precise for implementers.