feat: add overlap() to merge 2 FileStructure objects

[VulnX]

Closes: #2634

What does this PR add?

This PR adds an overlap() method to the existing FileStructure class which allows the user to merge 2 FileStructure objects. This is particularly useful when we have overlapping structs linked via the ->chain field.

Apart from the main overlap() method, for convenience, this PR also introduces 2 new methods:

1. from_bytes() - a class method which allows the user to create a FileStructure object from a byte string
2. offsets() - exports the architecture-specific offsets for all fields of the struct. This will also be very helpful in case the user wants to manually construct the fields with flat(), and use sane names like offsets['vtable'] instead of hard-coded numbers like 216

Does this PR modify any existing code?

No

[peace-maker]

Do you think this can be done with the generic overlap function from #2647? So we just keep the from_bytes and offsets helpers, but merging of file structs can be done on the bytes representation? Or do you have additional logic in your overlap function that needs to be preserved?

[peace-maker]

#2647 was merged. How can this PR be moved forward?

[VulnX]

Hey @peace-maker, sorry for the delay in response.
Yes I believe we can retain from_bytes and offsets while ensuring that overlap acts as a clean wrapper around the more generic version in #2647.

I will push the changes soon.

[Arusekk]

I believe this is a major redundancy with fit(), could these two APIs be made to fit [pun] each other?

[peace-maker]

This one about the file structure or the overlap in general? It seems they could be merged/aliased indeed.

[VulnX]

I believe this is a major redundancy with fit(), could these two APIs be made to fit [pun] each other?

I am not sure how this can be merged with flat(), since that relies on the assumption that data at different offsets do NOT overlap.

[VulnX]

I agree with @peace-maker though, that there should be only 1 definition for overlap in pwnlib/util/packing.py‎, and the one in FileStructure class should just be a wrapper around that.