Safe code only version

I made [version](https://github.com/jutuon/rustface/tree/safe) 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.

```none
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.

Start by comparing the safe branch with the unsafe implementation, then run the example program with the bundled test image and the detect_single_image/detect benchmark, with the rayon feature disabled. Done means the safe version's output remains identical and the project has a decided path for accepting or rejecting the performance tradeoff.

Written by the indexing model from the issue text.