Implement image loader (PNG, JPG ...)
This will probably be separated in a different package later.

Options include:
- stbimage [C]: https://github.com/nothings/stb/blob/master/stb_image.h
and [Nim]:
   - https://github.com/define-private-public/stb_image-Nim
   - https://github.com/acolley/nim-stb-image/blob/master/stbimage.nim
   - https://github.com/msiglreith/nim-stb_image
   - https://github.com/yglukhov/nimx/blob/master/nimx/load_image_impl.nim
   - https://github.com/ftsf/ld38-smalltrek/tree/master/src
- Porting PyTorch accimage [C+Python]: https://github.com/pytorch/accimage
- OpenCV [Nim]: https://github.com/dom96/nim-opencv
- Porting Pillow-SIMD [C+Python]: https://github.com/uploadcare/pillow-simd
- FreeImage [C]: http://freeimage.sourceforge.net/ (check license)
- DevIL [C++]: https://github.com/DentonW/DevIL
- Vapoursynth Image Reader [C+Python]: https://github.com/chikuzen/vsimagereader

Considerations:
- Can work on ARM
- Fast (AVX2 for x86-64 and Neon for ARM)
- Easy to integrate and maintain

Open question:
- For data augmentation like rotation/shearing, would it be better to use the library functions (tying it deeply with Arraymancer) or implement them directly on Arraymancer tensors (ensuring compatibility with all backends, and potentially using GPU acceleration)

Forum topic: https://forum.nim-lang.org/t/3056