* ENABLE_SPAN_T doesn't require UNSAFE_BYTEBUFFER.
Change to ENABLE_SPAN_T that doesn't require UNSAFE_BYTEBUFFER.
* Selectable framework.
Changed target framework to allow selection of 2.0 or 2.1 (or higher)
* Added target framework version check.
* Add core test project.
* Added run on .Net Core.
* [C#] Fix and improve project files
* "net35" target included for Unity 5
* "net46" target included for Unity 2017
* "netstandard2.0" target included for Unity 2018 and general use
* "netstandard2.1" target included for Span<T> support
Included project properties for defining UNSAFE_BYTEBUFFER, BYTEBUFFER_NO_BOUNDS_CHECK, and ENABLE_SPAN_T conditional compiler constants.
* Add documentation on building for C#