Fix no_std when the R1CS feature is on (#52)

Author: weikengchen

.github/workflows/ci.yml

@@ -117,5 +117,5 @@ jobs:
 
       - name: crypto-primitives
         run: |
-          cargo build --no-default-features --target aarch64-unknown-none
-          cargo check --examples --no-default-features --target aarch64-unknown-none
+          cargo build --no-default-features --features=r1cs --target aarch64-unknown-none
+          cargo check --examples --no-default-features --features=r1cs --target aarch64-unknown-none

src/encryption/elgamal/constraints.rs

@@ -10,7 +10,7 @@ use ark_ff::{
     fields::{Field, PrimeField},
     to_bytes, Zero,
 };
-use core::{borrow::Borrow, marker::PhantomData};
+use ark_std::{borrow::Borrow, marker::PhantomData, vec::Vec};
 
 pub type ConstraintF<C> = <<C as ProjectiveCurve>::BaseField as Field>::BasePrimeField;