Public access to the backing buffer uses Span<T> instead of ArraySegment<T>.
Writing to the buffer now supports Span<T> in addition to T[].
To maintain backwards compatibility ENABLE_SPAN_T must be defined.
Include C# codegen in flatc and .NET FlatBuffer access via the
FlatBufferBuilder class
Tested: on Windows.
Change-Id: If5228a8df60a10e0751b245c6c64530264ea2d8a