Overlap 2 file pointer structures for FSOP

[VulnX]

Often in CTFs, we may need to create 2 overlapping fake file structures and link them with the ->chain field.

The current implementation of the FileStructure class in pwntools is excellent for a single file struct, but it becomes difficult when we have overlapping structs.

Hence, it would be good if pwntools provided an automatic way to merge/overlap two FileStructure objects.

I believe I can implement this and would open a PR shortly

[RocketMaDev]

Wow, it seems that I didn't see such a issue and opened another pull request to implement overlap in #2647 .

Actually I appreciate your exploration on from_bytes, but as for overlap, my version, which is more generic may be better. For example, overlapping file, wide_data and jump_table together.

[VulnX]

Hey, your PR looks good. I see that it focuses on overlapping raw byte strings, which is a nice and generic approach.

Mine is pretty similar in purpose but works natively with the FileStructure class objects instead of raw bytes. Also, the 2 extra methods I added - from_bytes() and offsets() should be useful later on for the other parts of the module too.

I think both PRs could make sense in different contexts, but maybe @Arusekk can suggest what would be the best thing to do here.