Add operator trait constraints to std::num::{Zero, One} and document their appropriate use

Zero and One have precise definitions in mathematics. Documentation has been added to describe the appropriate uses for these traits and the laws that they should satisfy.

For more information regarding these identities, see the following wikipedia pages:

- http://wikipedia.org/wiki/Additive_identity
- http://wikipedia.org/wiki/Multiplicative_identity
Clean up std::num::pow implementation

This also removes the need for the Clone trait on T and improves the associated test coverage.