Dynamically allocated Array mutable but non resizeable

[Jalil David Salamé Messina]

I'm searching for a way to do something like this in D:

```cpp
struct MyStruct {
   const size_t length;
   int *const data;

   MyStruct(size_t n) : length(n) {
     data = new int[length];
   }
}
```

This way it is mutable, but non resizeable:
```cpp
MyStruct s = MyStruct(10);
s.data[0] = 42;        // Valid
s.data = new int[20];  // Error: assignment to const
```

Doing some extra reading about the theme I found the section on 
[Type Qualifiers](https://dlang.org/spec/const3.html), which 
makes me believe that the "tail constness" of D will disallow 
this kind of behavior.

My solution would be overriding the `[]` to make MyStruct behave 
as an array and hiding the implementation details, but if you 
have a more elegant solution I'm all ears!