to_bits_be?

[weikengchen]

Our current ToBitsGadget already has to_bits_le,

/// Specifies constraints for conversion to a little-endian bit representation of `self`.
pub trait ToBitsGadget<F: Field> {
    /// Outputs the canonical little-endian bit-wise representation of `self`.
    ///
    /// This is the correct default for 99% of use cases.
    fn to_bits_le(&self) -> Result<Vec<Boolean<F>>, SynthesisError>;

    /// Outputs a possibly non-unique bit-wise representation of `self`.
    ///
    /// If you're not absolutely certain that your usecase can get away with a
    /// non-canonical representation, please use `self.to_bits()` instead.
    fn to_non_unique_bits_le(&self) -> Result<Vec<Boolean<F>>, SynthesisError> {
        self.to_bits_le()
    }
}

Would it be useful to also add to_bits_be, which would be getting the result of to_bits_le and then reversing it? This is useful because legacy programs may have been using the old to_bits.