Durations in existing JDK APIs

[Martin Buchholz]

Obvious progress would seem to be more conversion methods.  Conversion code
tends to be annoying/errorprone because of having to deal with overflow.

Stephen/Doug: is there any reason we didn't add conversions between
Duration and TimeUnit when we added conversions to ChronoUnit?

Here's a strawman:

    /**
     * Converts the given time duration to this unit.
     *
     * @param duration the time duration
     * @return the converted duration in this unit,
     * or {@code Long.MIN_VALUE} if conversion would negatively overflow,
     * or {@code Long.MAX_VALUE} if it would positively overflow.
     */
    public long convert(Duration duration) {
        long s = convert(duration.getSeconds(), SECONDS);
        if (s == Long.MIN_VALUE) return s;
        long n = convert(duration.getNano(), NANOSECONDS);
        assert n >= 0 && n < 1_000_000_000;
        return (s + n < s) ? Long.MAX_VALUE : s + n;
    }