Preparation to make decoder use less memory and API more type safe.
Now each scalar type has it's own struct type so it can store different
things and enables to have a scalar interface.
Also own types will enable experimenting with decode DLS designs like
using chained methods that are type aware.
Refactor mp4 decoder to be simpler and have fallback for unknown box type
Cleanup some old ilst hacks
Add generic string reader to decode API that takes an encoding parameters
