Add Saturation trait to bevy_color

[boondocklabs]

Objective

• Allow for convenient access and mutation of color saturation providing following the Hue, Luminance traits.

• with_saturation() builder method

• saturation() to get the saturation of a Color

• set_saturation() to set the saturation of a mutable Color

Solution

• Defined Saturation trait in color_ops.rs
• Implemented Saturation on Hsla and Hsva
• Implemented Saturation on Color which proxies to other color space impls.
• In the case of colorspaces which don't have native saturation components
  the color is converted to 'Hsla` internally

Testing

• Empirically tested

---

Showcase

fn next_golden(&mut self) -> Color {
    self.current
        .rotate_hue((1.0 / GOLDEN_RATIO) * 360.0)
        .with_saturation(self.rand_saturation())
        .with_luminance(self.rand_luminance())
        .into()
}