interval/calculate

• clampInterval: Return the part of interval that lies inside bounds, or null when none of it does.
• intersectIntervals: Return the half-open interval two intervals share, or null when they share no instant.
• mergeIntervals: Collapse a list of half-open intervals into the fewest sorted, non-overlapping intervals.
• splitIntervalAt: Split a half-open interval at instant boundaries into consecutive half-open pieces.
• subtractIntervals: Return the parts of from that no interval in remove covers, as sorted half-open intervals.
• sumIntervals: Return the time covered by a list of half-open intervals, as an exact ISO 8601 duration.