C# support for directly reading and writting to memory other than byte[]. For exampl...

C# support for directly reading and writting to memory other than byte[].  For example, ByteBuffer can be initialized with a custom allocator which uses shared memory / memory mapped files. (#4886)

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.