Memory Mapped LoadState/SaveState

XML docs fixes

- A C# array cannot exceed 2,147,483,591 bytes. In my own use of LlamaSharp I encountered this limit.
 - This saves an extra copy of the entire state data into a C# `byte[]`, so it should be faster.

This does _not_ fix some other places where `GetStateData` is used. I'll look at those in a separate PR.

Adapt the interface change in llama_backend_init