Avoid allocation in sector roots

[ChrisSchinnerl]

I noticed that the inline type hints showed a Vec as an intermediary result. Apparently chunks requires a vector to pass on the references of step_by.
This PR fixes that to avoid that allocation and uses the recommended par_chunks_exact_mut instead of chunks.

     Running benches/merkle_root.rs (target/release/deps/merkle_root-9e0ec57c6e502a76)
Gnuplot not found, using plotters backend
sector_root             time:   [2.9715 ms 2.9886 ms 3.0070 ms]
                        change: [-8.0434% -6.9685% -6.0062%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 3 outliers among 100 measurements (3.00%)
  3 (3.00%) high mild