Planned breaking changes for v4.0

[pgrawehr]

For the next major release of Iot.Device.Bindings.dll and System.Device.Gpio.dll we intend to make the following breaking changes:

• Remove support for PinNumberingScheme. Support for this was only ever added and working for Raspberry Pi 3+, and even there it causes more harm than good by adding extra complexity and user confusion. Other boards never supported that feature. As an alternative, we will have a VirtualGpioController class that can be used for more generic pin mappings.
• Drop support for .netstandard. This simplifies logic to interface with native libraries, as we have access to NativeLibrary and other .NET core-only support functions. Since our most important target platform is linux, this would only affect people still using mono.
• Drop support for Windows 10 IoT. This was never really popular and is unmaintained.

Feel free to post a comment if you have objections or further cleanup ideas.