This PR makes vsvg::Unit and vsvg::Length first class citizen in whiskers:

• both can now be used as sketch parameters
• most Sketch APIs (from the Draw and Transform traits) accept now impl Into<f64> for spatial coordinate arguments, which means Unit and Length can directly be passed
• UI widget for Unit and Length support the #[param(all_units)] attribute to include large, typically irrelevant units (Unit::Km, etc.)
• the reduced list of units is now available as vsvg::SMALL_UNITS
• UI widget for Length further support the same set of parameters as numeric types (min, max, slider, log, etc.)

let u = self.unit;
sketch.line(u, 2. * u, 7. * u, 4. * u);
sketch.translate(0., 2. * u);
sketch.rect(4. * u, 2. * u, 6. * u, 2. * u);