[RFC] Convert identity functions in Field, Group, and {Projective,Affine}Curve traits with One/Zero traits from num_traits.

- contributes to #50,
- depends on #53 and builds on it,
- due to coherence & requirements of `num_traits::{Zero, One}` to implement `std::ops::Add<Self, ..>` and (resp.) `std::ops::Mul<Self, ..>`, I've had to replace the afferent `impl<'a, P: ..> (Add|Mul)<&'a Self> for Group(Affine|Projective)<P>` by direct implementations on `Self`,
- I did not have to fight the borrow checker for this conversion => I think this hints arithmetic operations are called in contexts where the operand is owned,
- hence should this end up on a merge track, we may want to open an issue to convert the `impl<'a, P:..> (Neg|Sub|..)<&'a Self> for ..<P>` trait usage to direct `impl<P:..> (Neg|Sub|..)<Self> for ..<P>`
- the `impl AddAssign for GroupAffine<P>` in curves/models/short_weierstrass_jacobian.rs is provided to fit trait bounds, and without any guarantee of suitability for any particular purpose
- and that, even though I don't think it's used.

crypto-primitives/src/signature/schnorr/mod.rs

@@ -6,6 +6,7 @@ use algebra::{
     to_bytes, ToConstraintField, UniformRand,
 };
 use digest::Digest;
+use num_traits::{One, Zero};
 use rand::Rng;
 use std::{
     hash::Hash,