Mutate immutable inside shared static constructor

[Nick Treleaven]

I've not used static constructors before, but it seems like the 
following should not be allowed:

```d
import std.stdio;

immutable int x;

@safe shared static this()
{
     x.writeln(); // 0
     x = 5;
     x.writeln(); // 5
     x = 6;
     x++;
     assert(x == 7);
}
```
Should I file a bug to require that `x` is only written to once? 
That would make it consistent with class constructors:

```d
class C
{
     immutable int x;
     this()
     {
         x = 5;
         x = 6; // error, x initialized multiple times
     }
}
```