Add .NET Core support

[DragoonAethis]

This lets dspatch run under Linux/macOS natively with .NET Core instead of Mono. Do dotnet build dspatch-core.csproj next to that .csproj and that's it.

The unsafe block initially threw a build warning and looked a bit magical so I quickly replaced it but it's not really required for anything, other than looking nicer - I can get rid of that if you'd prefer the original code.