EXT4: Expose more API surface

[dcantah]

The ContainerizationEXT4 package is extremely useful, but a lot of the API surface is internal or private. The APIs are all hit in the tar -> block device code paths, and most of them are just modeling the underlying filesystem layout so it should be somewhat easy (and ideal) to expose these for folks that want lower level access to these structures.

[dcantah]

Going to close this now that #112 exposes the superblock, but someone feel free to reopen if they want more out of that library