pub fn one_needle_byte(&self) -> anyhow::Result<u8> {
        anyhow::ensure!(
            self.needles.len() == 1,
            "benchmark only supports one needle, but {} were found",
            self.needles.len(),
        );
        anyhow::ensure!(
            self.needles[0].len() == 1,
            "needle must have length 1 (in bytes) but it has length {}",
            self.needles[0].len(),
        );
        Ok(self.needles[0][0])
    }
Return a single needle byte from this benchmark. If the benchmark has
no needles or more than one, then an error is returned. An error is
also returned if the single needle a length not equal to 1.
benchmarks/shared/lib.rs