Use Zero and One from num_traits

[kobigurk]

As described in https://github.com/AleoHQ/snarkOS/pull/219:

https://github.com/scipr-lab/zexe/ uses the Zero and One traits from the num_traits crate. Due to a trait resolution problem in Rust, the Add trait that the num_traits traits add to each implementer of Zero and One makes the current Add<&'a Self> unusable. While it seems like a good idea to do the same as Zexe, it requires wide code-base changes and so I'll leave that for a future PR. The traits introduced here are a version of the num_traits traits without the additional Add constraint.

[damons]

Team, is this something we want to do?

[ljedrz]

Moving to snarkVM in case it's still desired.