Make all types that implements `::num_traits::Zero` implement `crate::num_traits::Zero`

[TonalidadeHidrica]

This PR depends on #102 and #105. In addition to the changes applied in the PRs, this PR has a bonus: every type that implements ::num_traits::Zero implements crate::num_traits::Zero too. Therefore, if your crate depends on num_traits and have a type that implement ::num_traits::Zero and want to use it for Fenwick Tree, you can just use it without suffering. On the other hands, even fi your crate does not depend on num_traits but still want to use your own type for Fenwick Tree, you can manually implement crate::num_traits::Zero.