Function zerocaf::edwards::mul_by_pow_2

pub fn mul_by_pow_2<'a, T>(point: &'a T, _k: u64) -> T where
    &'c T: Mul<&'c Scalar, Output = T>, 

Compute ([2^k] * P (mod l)).

Note: The maximum pow allowed is 249 since otherways we will be able to get results greater than the prime of the sub-group.