Remove Copy from Ensure* traits (paritytech#13043)

* Remove Copy from EnsureOp and EnsureOpAssign

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

* Remove Copy from EnsureFrom and EnsureInto

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

* Fix default impl

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

* Reuse assignment code in Ensure trait

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

* Require Ensure for all BaseArithmetic types

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

* Fix assign impls

Co-authored-by: Luis Enrique Muñoz Martín <lemunozm@gmail.com>

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

* Add tests

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

* Add success doc tests

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>