* moved the sprite renaming out of the file system entirely into a caller-provided callback.
* renamed several functions to closer match the terms of a file system.
* moved the VM interface out of the implementation.
This is now being managed by the main file system class. The single lumps should only concern themselves with the actual data they manage, not with how the file system presents them to the outside.
The IWAD detection code was also switched to use a file system wrapper instead of looking at the single files directly.
