Add utils for PlainDate ranges, generators and sequences

[bjuppa]

If we start with a generator, we can use that to create some nifty range- and sequence-related functions:

• plainDateGenerator(next: (x: PlainDate) => PlainDate)(first: PlainDate): Iterable<PlainDate>
• plainDateRange(days: number)(first: PlainDate): PlainDate[] (days may be negative!)
• plainDatesBetween(first: PlainDate)(last: PlainDate): PlainDate[]
• firstPlainDate(predicate: (x: PainDate) => boolean))(sequence: Iterable<PlainDate>): PlainDate | undefined

See https://github.com/tc39/proposal-iterator-helpers