Reintroduce features that were left out from the initial release

[LukasKalbertodt]

I should do that one step at a time, to not get overwhelmed with work again. Some things are certainly more important than others.

• Core IO: traits, loading from PLY/STL, derives. Important I think. Reintroduce IO functionality #33
• Shapes: This requires IO and is very little code on top of it, so it should be easy to introduce that.
• Fuzzing tests: these are for IO. Might want to reintroduce them with IO?
• Face delegate mesh: I'm not even sure what that is, it was commented out a good while ago. I think it might be a new data structure I came up with. Not sure if it's any useful, but might be worth looking into.
• Benchmarks: Would be very useful to make them compile again and have a way of quickly extracting their data.
• Fat any mesh: I think only useful for loxi, so leave it for now.
• loxi: useful CLI tool for working with meshes. No high priority.