Rethink OOP access permissions

[Daniel N]

This is an alternative to "private(this)".

I think it would be much more interesting if it was possible to 
separate read and write permissions instead.
(The syntax is just an example, not that important, just for 
illustration purposes.)

```d
class C
{
     get(module) set(class) int x = 0;
}
```

Some ideas from C# are also interesting. They have 'init' and 
'readonly' aswell.

https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/proposals/csharp-9.0/init

```d
struct Point
{
     public int X { get; init; }
     public int Y { get; init; }
     public readonly int Z { get; init; }
}
```

Instead of trying to cater to a single usecase ("private(this)"). 
We could get unix like access permissions instead? Some 
additional benefits would be fast access to read-only variables 
instead of an expensive function call.