refactor: remove now-unused offset field in fat_extent
refactor: remove unneeded index field in fat_extent
refactor: replace extent_type with prev in fat_extent

This is in preparation for evaluating consistent chain state.
extent_type was no longer needed since only the distinction chain vs
literal was still relevant. This distinction is now encoded in the
prev value, which is 0 for literal chains.
[rw] Don't extend a chain with an invalid value
[rw] Add fat_is_consistent() call, not used yet

This call will later be used as part of a check if the whole image
is in a consistent state.

Also add some tests for writeable fat again