Pre-build DLL for C# bindings

[randomPoison]

Currently we drop the generated C# bindings for the shared Rust code directly into the Unity project. This is perfectly functional, but it has the annoying issue that when you use the Go To Definition tool in Visual Studio (or presumably any IDE), it jumps you into the very non-human-readable generated bindings. It would be better if it took you to the metadata view instead, since that view is still readable no matter how the source code is formatted. We can force this if we instead build the generated bindings into a DLL and then copy the built DLL into the Unity project.