Port to .NET Core

[ekolis]

Why? Because. Oh yeah, Xamarin. #32

[ekolis]

This will require rewriting the Eggs module, or decompiling it and recompiling it; I seem to have lost the source code for that DLL.

[ekolis]

.NET 5 is coming in November; this might be a moot point... just upgrade to that instead and it we'll get the best of both worlds!