.NET 5 support
Problem to solve
Provide .NET 5 build, or at least verify/test current TFM work fine with .NET 5.
.NET 5 is scheduled to be released in November 2020. https://github.com/dotnet/core/blob/master/roadmap.md#upcoming-ship-dates
Test, add CI.
We could do some interesting things with the new Native Hosting API
Just a remark, that's out of scope from this ticket.