[C#] Fix compile issue when compiling with older versions of C# (#4938)

* 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.

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.

* Remove usage of expression bodied method so that ByteBuffer can be compiled with older version of C#.

net/FlatBuffers/ByteBuffer.cs

@@ -116,7 +116,10 @@ namespace FlatBuffers
         }
 
 #if !ENABLE_SPAN_T
-        public override byte[] ByteArray => _buffer;
+        public override byte[] ByteArray
+        {
+            get { return _buffer; }
+        }
 #endif
 
 #if UNSAFE_BYTEBUFFER