expose all box types

[cTopher]

Instead of only exposing Mp4Box type, expose all box types to consumers.

[alfg]

Hey @cTopher, sorry for the late response and thanks for the PR. Any context behind exposing all the box types?

[Serial-ATA]

@alfg The box types are exposed in public fields of the exported types. Mp4Track for example has 2/3 of its fields with private box types, so you can't see their structure in the docs.

pub struct Mp4Track {
    pub trak: TrakBox, // private
    pub trafs: Vec<TrafBox>, // private
    pub default_sample_duration: u32,
}