Immutable/readonly structs (e.g. NSRange)

This is not really a bug, but more of an open discussion about how some things are bound.

Take for example, NSRange: https://github.com/xamarin/xamarin-macios/blob/master/src/Foundation/NSRange.cs#L27

The struct is guidelines are for the struct to be immutable. (https://docs.microsoft.com/en-us/dotnet/standard/design-guidelines/struct)

Cocoa API doesn't seem to let it be mutated either: https://developer.apple.com/documentation/foundation/nsrange?language=objc

In case of structs, when a struct is in a readonly field/property or is passed as arguments to `in` parameters, the compiler generates a defensive copy.

This happens because `this` is mutable by default.

C# 7.2 has some nice-ness that allows for structs to be marked as readonly, thus not allowing the `this` variable to be set, but that requires the property to be fully readonly.

Why was NSRange designed mutable? Might be the case for most structs bound