Remove some redundancy from the DValue hierarchy: all subclasses had a 'type' field and identical implementations of virtual function getType(). Move these to DValue itself, and make getType() non-virtual.

module tangotests.mem3;

void main()
{
    int[] arr;
    arr ~= [1,2,3];
    assert(arr[0] == 1);
    assert(arr[1] == 2);
    assert(arr[2] == 3);
    delete arr;
    assert(arr is null);
}