Remove unused num-traits dep

[RobWalt]

Hi, thanks for joining on polishing this new crate 😄

The num-traits dep is currently

• imported here https://github.com/georust/geo-svg/blob/frewsxcv-patch-1/src/svg_impl.rs#L6
• and as far as I can tell used here https://github.com/georust/geo-svg/blob/frewsxcv-patch-1/src/svg_impl.rs#L31-L35

I think we could get away with normal casting to f32 via as f32. If we want to go the extra safe route, we could use f64 instead of f32 and then cast via as f64 as that's more likely not to fail in weird ways 🤔

[frewsxcv]

I must have had a typo when I searched! Disregard