Simplifying finish_into

    /// This method will fail if the buffer is not large enough for

    /// the signature, one can use `finish_len` to get an upper bound

    /// on the required size.

    pub fn finish_into<B:AsMut<[u8]>>(&self, mut buf: B) -> Result<usize, ErrorStack> {

    pub fn finish_into(&self, buf: &mut [u8]) -> Result<usize, ErrorStack> {

        unsafe {

            let buf = buf.as_mut();

            let mut len = buf.len();

            cvt(ffi::EVP_DigestSignFinal(

                self.md_ctx,