v0.18.0

Changed

• Breaking: EventType.Duration changes type from uint64 to *uint64, so that an
  absent and a zero Event@duration stay distinguishable (see Fixed below). Setting it
  needs mpd.Ptr(uint64(d)), and reading it needs a nil check: nil now means the
  duration is unknown, which is not the same as zero.

Fixed

• An Event@duration of zero can now be expressed. ISO/IEC 23009-1 gives an absent
  @duration the meaning "the duration is unknown", which is not the same as a known,
  zero-length event, but with a plain uint64 and omitempty the zero was dropped on
  marshal and the two cases were indistinguishable on unmarshal. SCTE 214-1 §6.7.2.1
  relies on the difference: an SCTE-35 event that closes an earlier one "should have a
  duration of zero and shall not be infinite".