Use spans to improve performance in StreamExts.

Also avoid ReadBytes calls that allocate a buffer by either updating the stream position (if not interested in the bytes), by reusing an input buffer (if interested in the bytes), or using a stackalloc buffer to avoid the allocation (for small reads).

OpenRA.Game/Network/ReplayRecorder.cs

@@ -67,7 +67,7 @@ namespace OpenRA.Network
 				}
 			}
 
-			file.WriteArray(initialContent);
+			file.Write(initialContent);
 			writer = new BinaryWriter(file);
 		}
 
@@ -93,7 +93,7 @@ namespace OpenRA.Network
 		{
 			var ms = new MemoryStream(4 + data.Length);
 			ms.Write(frame);
-			ms.WriteArray(data);
+			ms.Write(data);
 			Receive(clientID, ms.GetBuffer());
 		}