Consider `num_traits`

[Arjentix]

I wanted to go with this crate and your bls128_381 crate. Everything was fine until I tried to combine it with ndarray where I can't do dot() operations and etc because of unsatisfied trait bounds like One.

From my point of view it makes sense for ndarray to use num-traits as it's most common way to have more specialized number related things that Rust doesn't provide from the box. So I'd expect that this ff crate also use this for super-trait bound on Field trait.

For now I think I must switch to ark-ff