Safe code only version

[jutuon]

I made version of this library with safe code only. The example program produces exactly the same output image as the version with unsafe code using the bundled test image. The performance is unsuprisingly a bit worse.

MacBook Air M1 2020 and library's rayon feature is disabled.

Commit 271563b, safe code only:
detect_single_image/detect
                        time:   [334.29 ms 334.36 ms 334.44 ms]

Commit fa3d5b5, includes unsafe:
detect_single_image/detect
                        time:   [327.55 ms 327.83 ms 328.05 ms]

I think next step to optimize the safe version would be to check if nalgebra could be used in some places to make calculations use SIMD. However, I don't plan to optimize the code any further.

I didn't open PR for this because of the performance decrease, but I can open it if wanted.