Restrict std::num::{Zero, One} implementors and improve std::num::pow implementation

`Zero` and `One` have precise definitions in mathematics as the identities of the `Add` and `Mul` operations respectively. As such, types that implement these identities are now also required to implement their respective operator traits. This should reduce their misuse whilst still enabling them to be used in generalized algebraic structures (not just numbers). Existing usages of `#[deriving(Zero)]` in client code could break under these new rules, but this is probably a sign that they should have been using something like `#[deriving(Default)]` in the first place.

For more information regarding the mathematical definitions of the additive and multiplicative identities, see the following Wikipedia articles:

- http://wikipedia.org/wiki/Additive_identity
- http://wikipedia.org/wiki/Multiplicative_identity

Note that for floating point numbers the laws specified in the doc comments of `Zero::zero` and `One::one` may not always hold. This is true however for many other traits currently implemented by floating point numbers. What traits floating point numbers should and should not implement is an open question that is beyond the scope of this pull request.

The implementation of `std::num::pow` has been made more succinct and no longer requires `Clone`. The coverage of the associated unit test has also been increased to test for more combinations of bases, exponents, and expected results.