Oops, I accidentally implemented the biggest parts of `ISO/IEC 14496-12:2015(E)`

[onkoe]

I accidentally implemented a general MPEG-4/BMFF parser for HEIF.

Since the parsers within are MUCH better than my old ones (which were temporary anyway), we should move all the shared stuff into the bmff/ module and shrink heif/ into heif.rs.

Checklist

Dang, this'll take a bit...

• Move shared behavior into bmff submodules
• Replace godawful bmff parsers with the new shared behavior
• Make heif module into a single heif.rs file
• Rewrite mp4.rs and mov.rs to utilize shared parsing structures
  • In other words, since we can better parse concepts like superclasses and items, use those parsers inside MP4 to be more flexible.