Basic metadata on chunks & extracted files

[martonilles]

As limited meta-data report, we want to report on chunks & extracted files to represent the layers of a firmware image.
We need to represent the relationship between files & chunks, but do not need details like file permission/owner, additional details from chunks etc. Those can be added later in #16.

Our goal is to export the following as a first milestone:

FilesystemObject

• parent (Chunk, null for root)
• children (list of Chunk, could be zero)
• path
• type (File, Directory, Device, Symlink etc.)
• size
• file magic (if available)

Chunk

• parent (File)
• children (list of Files, could be zero)
• start/end offset
• length
• type (handler)

Include any extraction/processing error.
Store the result in a json (or similar machine readable file, which can be later extended with additional meta-data)