Fix constness of array.d

Thanks to @aG0aep6G this works as far as I can see.
Changing to inout triggers a compiler bug and errors as

```
array.d(235,22): Error: variable std.container.array.Array!int.Array.RangeT!(inout(Array!int)).RangeT._outer_ only parameters or stack based variables can be inout
array.d(235,22): Error: variable std.container.array.Array!int.Array.RangeT!(inout(const(Array!int))).RangeT._outer_ only parameters or stack based variables can be inout
array.d(333,9): Error: template instance std.container.array.Array!int.Array.RangeT!(inout(const(Array!int))) error instantiating
array.d(328,9):        instantiated from here: RangeT!(inout(Array!int))
array.d(386,19):        instantiated from here: RangeT!(Array!int)
array.d(898,5):        instantiated from here: Array!int
array.d(333,9): Error: template instance std.container.array.Array!int.Array.RangeT!(const(Array!int)) error instantiating
array.d(386,19):        instantiated from here: RangeT!(Array!int)
array.d(898,5):        instantiated from here: Array!int
array.d(927,5): Error: static assert  (!true) is false
```