Function verify_prehashed 

pub fn verify_prehashed<C>(
    pubkey: <C as IntrinsicCurve>::Point,
    prehash: &[u8],
    sig: &[u8],
) -> Result<()>
where
    C: IntrinsicCurve + PrimeCurve,
    C::Point: WeierstrassPoint + CyclicGroup + FromCompressed<<<C as IntrinsicCurve>::Point as WeierstrassPoint>::Coordinate>,
    <<C as IntrinsicCurve>::Point as WeierstrassPoint>::Coordinate: IntMod,
    C::Scalar: IntMod + Reduce,
    for<'a> &'a C::Point: Add<&'a C::Point, Output = C::Point>,
    for<'a> &'a <C as IntrinsicCurve>::Scalar: DivUnsafe<&'a <C as IntrinsicCurve>::Scalar, Output = <C as IntrinsicCurve>::Scalar>,

Assumes that sig is proper encoding of r, s.