Copying array with const correctness

[Vindex9]

Here's a slightly better solution. The lines will be copied.

```d
T[] copyArray(T)(inout(T)[] arr) {
     T[] copy = new T[arr.length];
     copy.length = arr.length;
     static if (is(T == U[], U) && !is(T == immutable(Y)[], Y)) {
         foreach(i, ref v; copy) {
             v = copyArray(arr[i]);
         }
     } else {
         for (size_t i = 0; i < arr.length; i++) {
             static if (is(T == immutable(Y)[], Y)) {
                 copy[i] = arr[i].idup;
             } else {
                 // it doesn't work with postblit, only with 
modern copy ctor
                 T elem = arr[i];
                 copy[i] = elem;
             }
         }
     }
     return copy;
}
```