Support Interval#translate and Interval#EMPTY

[glains]

I would propose to enhance the current Interval implementation.

First, Interval.EMPTY, as an addition to the already existing Interval.ALL. The current approach requires to pass the same interval twice, such as Interval.of(Instant.MIN, Instant.MIN).

/**
 * An empty interval.
 */
public static final Interval EMPTY = new Interval(Instant.MIN, Instant.MIN);

Secondly, a method to tralslate the current interval using the already existing Duration class. Take the following example:

Duration translation = Duration.ofMinutes(2);
interval.withStart(interval.getStart().plus(translation))
    .withEnd(interval.getEnd().plus(translation));

The given translation can be negative to support either plus or minus.

An implementation might look like this:

public Interval translate(Duration translation) {
    requireNonNull(translation, "The translation must not be null");
    return new Interval(start.plus(translation), end.plus(translation));
}

One could also check for Duration.isZero() before and return the same instance.