[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/Cargo.toml

@@ -32,6 +32,7 @@ bench-utils = { path = "../bench-utils" }
 digest = "0.7"
 blake2 = "0.7"
 
+num-traits = { version = "0.2.11" }
 rand = { version = "0.7" }
 derivative = "1"
 rayon = "1"

crypto-primitives/src/merkle_tree/mod.rs

@@ -372,6 +372,7 @@ mod test {
         merkle_tree::*,
     };
     use algebra::curves::jubjub::JubJubAffine as JubJub;
+    use num_traits::Zero;
     use rand::SeedableRng;
     use rand_xorshift::XorShiftRng;
 
@@ -419,7 +420,6 @@ mod test {
     }
 
     fn bad_merkle_tree_verify<L: ToBytes + Clone + Eq>(leaves: &[L]) -> () {
-        use algebra::groups::Group;
         let mut rng = XorShiftRng::seed_from_u64(13423423u64);
 
         let crh_parameters = Rc::new(H::setup(&mut rng).unwrap());

crypto-primitives/src/nizk/mod.rs

@@ -59,7 +59,8 @@ mod test {
     #[test]
     fn test_gm17() {
         use crate::nizk::{gm17::Gm17, NIZK};
-        use algebra::{curves::bls12_381::Bls12_381, fields::bls12_381::Fr, Field};
+        use algebra::{curves::bls12_381::Bls12_381, fields::bls12_381::Fr};
+        use num_traits::One;
         use r1cs_core::{ConstraintSynthesizer, ConstraintSystem, SynthesisError};
 
         #[derive(Copy, Clone)]

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,