[C#] Improve Span<> utilization (#8588)

There are a couple instances where the ByteBuffer's Span property was accessed in a loop.
 + Extracted the use of the property outside of the loop to save a few cpu cycles.

Access to the allocator's internal buffer isn't exposed as a ReadOnlySpan<byte> from the ByteBuffer
or the FlatBufferBuilder.
 + Added a few convenience functions to access the buffer using a ReadOnlySpan<byte>.

There are a few cases where built in Span extensions can be used to run optimized code.
 + Added the use of Span.Fill() and ReadOnlySpan.SequenceCompareTo to replace existing loops.

Co-authored-by: Björn Harrtell <bjornharrtell@users.noreply.github.com>
Co-authored-by: Wouter van Oortmerssen <aardappel@gmail.com>

net/FlatBuffers/FlatBufferBuilder.cs

@@ -957,6 +957,21 @@ namespace Google.FlatBuffers
             return _bb.ToSizedArray();
         }
 
+#if ENABLE_SPAN_T && UNSAFE_BYTEBUFFER
+        /// <summary>
+        /// A utility function return the ByteBuffer data as a
+        /// `ReadOnlySpan<byte>`.
+        /// </summary>
+        /// <returns>
+        /// A `ReadOnlySpan<byte>` that references the internal 
+        /// ByteBuffer data.
+        /// </returns>
+        public ReadOnlySpan<byte> SizedReadOnlySpan()
+        {
+            return _bb.ToSizedReadOnlySpan();
+        }
+#endif
+
         /// <summary>
         /// Finalize a buffer, pointing to the given `rootTable`.
         /// </summary>