Add additional range/interval operators

[erikc5000]

Some of the following operators could be quite useful additions:

• range1 union range2
• range1 intersect range2
• range1 span range2
• range1.abuts(range2)
• range1.encloses(range2)
• range1.overlaps(range2)
• range.mapEndpoints(start = { it.startOfWeek }, endInclusive = { it.endOfWeek })
• range.mapEndpoints { it.startOfWeek } - apply a function to both endpoints
• range.shiftedBy((-1).days) - specialization of mapEndpoints that maps the start/end by a period or duration
• range.copy(start = anotherDate)

ZonedDateTimeInterval and OffsetDateTimeInterval pose some challenges since the endpoints may have different zones or offsets. It might make sense to have them return InstantInterval if you try to perform a union or intersection on them, for example.