Support RIFF structures with variable structs

[200sc]

We need to be able to recognize when a field in a RIFF structure is supposed to be the length for a different field in the same RIFF structure.

[200sc]

This would probably need to happen through a new struct tag like "riffSize: $otherField", or maybe you'd just know it's the size of the next field (as that's almost always the case anyway).

Alternatively, the field themselves would say "riffSize: $sizeTag" and you wouldn't need to store those size fields on the structs in the first place, but that would only work if the individual fields are already tagged in the RIFF, which isn't often true