Revamp `unpack` to use `position`

[nvs]

Basically, let unpack in the various file formats take an additional position argument like string.unpack does. They also need to return the updated position as well. The assertion will be removed. The intention of that particular assert was to catch updates to file formats or other issues. Unit testing should probably be implemented to catch those issues (cough #53).

• [x] WTS
• [x] INI
• [x] Objects
• [x] SLK
• [x] W3O
• [x] Units DOO
• [x] Doodads DOO
• [x] Imports
• [x] Cameras
• [x] Terrain
• [x] Information
• [x] Regions
• [x] Sounds
• [x] Pathing

[nvs]

This issue probably does not need to be addressed. All of the supported file formats are processed as full files. The exception, and what prompted consideration of this change, is W3O support. Its format is a bunch of object modification files shoved together.

Instead of adjusting every file format to work in a way that will never be needed, it is probably best to do one of the following:

• Split the input string of the W3O file a bunch of times.
• Adjust the private object handling to take additional information (e.g. a position).