Get rid of SharpZipLib

There are some commercial users (all out of a sudden!) who could really use the Fenrir library, but they cannot due to us using SharpZipLib.

We've investigated a bit, and SharpZipLib is only required to battle with certain idiosyncrasies of deflate compression support in .NET.

We should take a look at removing this support, and replacing it with a `ZlibStream` (also part of .NET, more low-level though) or something else.