"Model 2" prototype status

[Simon Ochsenreither]

> I don't know why mutability of structs in .NET keeps being referred to as a
> mistake.  Sure it can lead to bugs, but so can many other things if not used
> properly.  It's a useful tool to have in some occasions (given .net supports
> passing them by ref).
> 

>From my perspective mutating value types is like changing the values inside of
Integer.IntegerCache. No need to have mutable value types when you can place
them at a writable location, especially because it wouldn't be possible to tell
anymore what code does without reading the definition of every class, like in
http://oxnrtr.de/scaladays2015/#46 .