optimise scanning code

[tbillington]

This code manually iterates over walkdir::WalkDir from the walkdir crate to intelligently skip directories and files under certain conditions using skip_current_dir.

Criterion results

Before

scanner_bench           time:   [5.5149 s 5.6332 s 5.7884 s]

After

scanner_bench           time:   [135.92 ms 136.49 ms 136.96 ms]
                        change: [-97.578% -97.535% -97.499%] (p = 0.00 < 0.05)
                        Performance has improved.

Results in a 97.5% reduction in scan runtime on my pc.