I updated the POC.

You can now declare the move constructor & move opAssing like the
following:

```
struct S
{
     this(ref S) @move {}     // move constructor
     opAssign(ref S) @move {} // move opAssign
}
```

Or with rvalue ref:

```
struct S
{
     this(@rvalue ref S) {}     // move constructor
     opAssign(@rvalue ref S) {} // move opAssign
}
```

All implementations use rvalue ref internally. It's just a matter
of exposing in the language it or not.