Fix involuntary conversions of ExtMove to Move

Fix involuntary conversions of ExtMove to Move

The trick is to create an ambiguity for the
compiler in case an unwanted conversion to
Move is attempted like in:

    ExtMove m1{Move(17),4}, m2{Move(4),17};

    std::cout << (m1 < m2) << std::endl; // 1
    std::cout << (m1 > m2) << std::endl; // 1(!)

This fixes issue #1204

No functional change.