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