Migrate BackrollSessionCallbacks into an interface or abstract class

[james7132]

The C style of passing callbacks works for a C/C++ implementation like GGPO with a C BPI, but does not work well in a C# environment. It might be better to set up a interface or abstract class. There are several benefits of this:

• No functor overhead via using delegates.
• Compile time enforcement of correctness, and improper assignment at runtime is impossible.
• Some of the callbacks take structs passed in as refs. Delegates will take non-ref arguments as valid matches, which will break saving/loading functionality.

[nykwil]

On point one, the functor overhead is the same as a virtual call. http://kennethxu.blogspot.com/2009/05/strong-typed-high-performance_15.html

[james7132]

At this rate, please use UnityGGPO, I'm more focused on maintaining the Rust version of this library than the Unity C# one.