Uniform randomization for field elements

[hackaugusto]

This enables uniform randomization for field elements, taking in consideration the field modulus so that every element has equal probability.

When compiled with the feature rand, this enables the following:

let a: CubeExtension<BaseElement> = rand::thread_rng().gen(); // <-- use of `gen()`
let expected = a * a;
assert_eq!(expected, a.square());