Rewrite elf serialization from xor to BytesIO

[DCNick3]

• This implementation errors out if any intersecting regions are detected instead of silently xor'ing them. (Is there even point in doing anything else?)
• It is much faster due to lower time complexity: O(n) vs O(n*m)

It addresses the same issue as #15 does, but without adding any external dependencies, changing the algorithm instead