Use inout for ref returning members in std.container.array

For motivation see 

http://forum.dlang.org/thread/agyeejbrpotpjrecepyl@forum.dlang.org#post-wysehvbsfnezfozsagpl:40forum.dlang.org
This

```D
inout(Range) opSlice() inout
{
    return inout(Range)(this, 0, length);
}
```

makes the

```
foreach (e; r)
```

fail as

```
array.d(862,5): Error: mutable method std.container.array.Array!int.Array.Range.popFront is not callable using a const object
```

Should I remove the `foreach`?