To simplify the build process (immensely), I'd like to move the CLR loading for Mono and CoreCLR to Python. I have a working prototype at https://gist.github.com/filmor/cf8a02aefff58be09a57ad2eca3cc246 for CoreCLR using CFFI and will work on an equivalent Mono implementation for this as well. For .NET Framework, I don't know of any public API to embed it into the currently running process, so we'll keep the existing loader for this one.

This implements the first section of the Plan(tm) (https://github.com/pythonnet/pythonnet/wiki/Plan#clr-loading).