Publish nested types

[kixelated]

Hey there,

The top level box types from the mp4box module are public: MoovBox, MoofBox, MdatBox. However, with the exception of EMsgBox, none of the nested types are public. This makes it difficult to initialize these structs.

For example, here's my awful code to fragment a MoovBox:

// We need to create a brand new moov atom for each track.
let mut toov = mp4::MoovBox{
  mvhd: moov.mvhd.clone(),
  meta: moov.meta.clone(),
  mvex: Some(Default::default()),
  traks: vec![trak.clone()],
  udta: moov.udta.clone(),
};

// TODO someone who knows Rust, please double check that I'm not dumb.
// MvexBox and TrexBox are not exported by the mp4 crate, so we have to construct them using Default.
toov.mvex.as_mut().map(|mvex| {
  mvex.trex = Default::default();
  mvex.trex.track_id = track_id;
});

If these types didn't derive Default, I'm not even sure that it would be possible to set them. Could you export these nested types, or change the mp4box box so it uses pub instead of pub(crate)?