Handle Building/testing netstandard-based PCL

[masojus]

Once we have a better idea of what will fit into the netstandard project and what won't, decide on how the solution will be structured, and how best to build and test, making the appropriate changes to the CI setup.

This may involve building/testing portions of the solution with the dotnet cli and others with msbuild or structuring it such that it all builds with one or the other or both.

https://docs.microsoft.com/en-us/dotnet/core/porting/project-structure https://oren.codes/2017/01/04/multi-targeting-the-world-a-single-project-to-rule-them-all/

[masojus]

This is basically done as part of the work in PRs #124, #125, and #126