Reimplementation and modernization of parts of unp4k

[diogotr7]

Hello, I'm making this issue as a thanks for your work mostly, and also as a way to let you know I've decided to port unp4k and unforge to modern c# versions, making them faster and hopefully cleaner since i have your work to reference.

Here's your changes to SharpZipLib required to extract unp4k's, with clean git history and rebased on the newest SharpZipLib codebase: https://github.com/diogotr7/SharpZipLibP4k

Here's my rewrite of unp4k's DataForge decoder, p4k extractor, and hopefully some sort of gui at some point in the future? https://github.com/diogotr7/StarBreaker

Again, thank you for all your findings, i had some trouble reimplementing things with the reference, I cannot image how head scratching it must have been the first time around :)