Add github CI, remove Travis/Appveyor, fix some clippy warnings, format code

[qarmin]

CI pass on fork - https://github.com/qarmin/printpdf/actions/runs/6362382828, and will pass also here but only after merging, because now repo still want to use appveyor

[fschutt]

I think this was superseded by #170, so closing it in favor of that one. The rest is just reformatting, which should have its own PR because it's harder to read otherwise.