Function random_mod_vartime_core 

pub(super) fn random_mod_vartime_core<T, R: TryRng + ?Sized>(
    rng: &mut R,
    x: &mut T,
    modulus: &NonZero<T>,
    n_bits: u32,
) -> Result<(), R::Error>
where
    T: Encoding + CtLt,

Generic implementation of random_mod_vartime which can be shared with BoxedUint.