Overlap 2 file pointer structures for FSOP

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