Rebooting the __metada/__mutable discussion

[rikki cattermole]

Here is my test code that I used to determine what I needed to do to 
make const RC types useless (and hopefully safe).

```d
void main() {
     auto r1 = /+const+/ Reference(2);
     const r2 = r1;
     const r3 = Reference(2);
     const(Reference) r4 = Reference(2);
     //auto r5 = cast(const)r1;
     auto r6 = cast(void*)&r1;
     auto r7 = cast(const)&r1;
}

struct Reference {
     int x;

@safe:

     this(int value) {
         this.x = value;
     }

     this(ref Reference other) {
         this.x = other.x;
     }


     ~this() {
         this.x = 0;
         writeln("destructor");
     }

     void opAssign(ref Reference other) {
         __ctor(other);
     }

     void opAssign(Reference other) {
         __ctor(other);
     }

     // bool isNull() { ... }

     @disable this(int value) const;

     //@disable this(ref Reference other) const;
     @disable this(ref const Reference other) const;
     @disable this(this);

     //@disable ~this() const;

     @disable void opAssign(ref Reference other) const;
     @disable void opAssign(Reference other) const;

     @disable auto opCast(T)();
}
```