convert nonnative constant to little endian (#127)

Author: slumber

CHANGELOG.md

@@ -3,6 +3,7 @@
 ## Pending
 - [\#117](https://github.com/arkworks-rs/r1cs-std/pull/117) Fix result of `precomputed_base_scalar_mul_le` to not discard previous value.
 - [\#124](https://github.com/arkworks-rs/r1cs-std/pull/124) Fix `scalar_mul_le` constraints unsatisfiability when short Weierstrass point is zero.
+- [\#127](https://github.com/arkworks-rs/r1cs-std/pull/127) Convert `NonNativeFieldVar` constants to little-endian bytes instead of big-endian (`ToBytesGadget`).
 
 ### Breaking changes
 

src/fields/nonnative/field_var.rs

@@ -313,7 +313,7 @@ impl<TargetField: PrimeField, BaseField: PrimeField> ToBytesGadget<BaseField>
     fn to_bytes(&self) -> R1CSResult<Vec<UInt8<BaseField>>> {
         match self {
             Self::Constant(c) => Ok(UInt8::constant_vec(
-                c.into_bigint().to_bytes_be().as_slice(),
+                c.into_bigint().to_bytes_le().as_slice(),
             )),
 
             Self::Var(v) => v.to_bytes(),
@@ -324,7 +324,7 @@ impl<TargetField: PrimeField, BaseField: PrimeField> ToBytesGadget<BaseField>
     fn to_non_unique_bytes(&self) -> R1CSResult<Vec<UInt8<BaseField>>> {
         match self {
             Self::Constant(c) => Ok(UInt8::constant_vec(
-                c.into_bigint().to_bytes_be().as_slice(),
+                c.into_bigint().to_bytes_le().as_slice(),
             )),
             Self::Var(v) => v.to_non_unique_bytes(),
         }