Non-const std.container.Array.opIndex and alikes blocks serialization

[Nordlöw]

In my strive to add msgpacking support for std.container.Array 
I've been blocked by the fact that std.container.Array.opIndex 
only has one overload which is non-const probably because it 
returns a ref T. This doesn't place nicely with serialization.

Shouldn't inout be used here?