Lift self-copy protection up to the header file and add self-move
protection to the same layer.

This is in line with Howard's advice on how best to handle self-move
assignment as he explained on SO[1]. It also ensures that implementing
swap with move assignment continues to work in the case of self-swap.

[1]: http://stackoverflow.com/questions/9322174/move-assignment-operator-and-if-this-rhs