Make ToBitsGadget for FpVar's output constant-length (#12)

This is related to a Zexe issue https://github.com/scipr-lab/zexe/issues/289

Basically, ToBitsGadget for FpVar may produce shorter output for a constant value, which is undesired for a few reasons.

src/fields/fp/mod.rs

@@ -876,7 +876,8 @@ impl ToBitsGadget for FpVar {
     fn to_non_unique_bits_le(&self) -> Result<Vec<Boolean<F>>, SynthesisError> {
         use ark_ff::BitIteratorLE;
         match self {
-            Self::Constant(c) => Ok(BitIteratorLE::without_trailing_zeros(&c.into_repr())
+            Self::Constant(c) => Ok(BitIteratorLE::new(&c.into_repr())
+                .take((F::Params::MODULUS_BITS) as usize)
                 .map(Boolean::constant)
                 .collect::<Vec<_>>()),
             Self::Var(v) => v.to_non_unique_bits_le(),