C# backend

[philpax]

We should prove that multiple (real) backend support is possible by adding a C# backend. I haven't written C# in a long while, and we'd need a project to prove it out, but it should be possible. (Some things may need to be fixed first: #47 #10 come to mind)

[philpax]

Another complication would be the heavy use of Rust-specific extern types, like SharedPtr (although that might work, given some similarity in the generic syntax). For this, we would either have to do #3 or #49.