v2.0.0

Changed

• Breaking: IndefiniteLoader.init(clock:delay:minimumDuration:timeout:) takes
  any Clock<Duration>, defaulting to ContinuousClock(). The loader measures how long its
  indicator has been visible in the clock's own instants.
• Breaking: MockClock is a standard library Clock with its own MockClock.Instant. A sleep
  parks until advance(by:) moves the reading to its deadline and returns at once when the
  deadline has already been reached, so a delay: .zero loader shows its indicator without the
  test advancing anything. now is a property, and init() replaces init(now:).
• Breaking: MockClock.advance(by:) traps on a negative duration. The reading never moves
  backward, as the Clock contract requires.

Removed

• Breaking: ClockProtocol and SystemClock. Pass any Clock measured in Duration;
  ContinuousClock is the default.
• Breaking: MockClock.advanceAllSleeps(). Advance the reading to a sleep's deadline instead;
  releasing a sleeper before its deadline would contradict the clock's reading.