Consider promoting `Field::{zero, one}` to associated constants

[tarcieri]

Right now Field::zero() and Field::one() are static methods of the Field trait.

If they were instead associated constants, e.g. Field::ZERO/Field::ONE, they can be used in const contexts such as defining other constants:

pub struct ProjectivePoint<Fe: Field> { x: Fe, y: Fe, z: Fe }

impl<Fe: Field> ProjectivePoint<Fe> {
    pub const IDENTITY: Self = Self {
        x: Fe::ZERO,
        y: Fe::ONE,
        z: Fe::ZERO,
    };
}

[str4d]

I vaguely recall that at the time the traits were first written, there were limitations on how the associated constants could be either constructed or used? I don't recall the specifics, but the course of action depends on several things:

• If all trait implementors will always be able to instantiate the associated constants, then we can add them.
• If all trait users will be able to use the associated constants everywhere they can use the trait methods (which definitely seems like it should be true for the versions of Rust we support), then we can remove the trait methods.

[str4d]

I've opened #94 so we can test the first bullet point.