NativeAOT support

[ceciliachan1979]

For people who might be interested/needed to get this to run under NativeAOT, I have a prototype built here, feel free to check it out.

You are warned! This change supports only a few APIs necessary for my work and it is full of warnings.

I don't plan to support it personally - so you are on your own if you want to use my fork - but I can accept pull requests, especially for those that support more APIs.

Official support for that would be really nice, although I notice by changing the libraries I can't support net45 anymore.