Design a non-ad-hoc way for color-to-color conversions

[jdahlstrom]

Currently there's an assortion of conversion methods, including:

• to_rgba for Color3<Rgb> -> Color3<Rgba>
• to_color3 for Color3f<Rgb> -> Color3<Rgb>
• to_hsl for Color3<Rgb> -> Color3<Hsl>
• to_linear for Color3f<Rgb> -> Color3f<LinRgb>

and more.

Figure out a way to make this less ad-hoc and prone to forgetting some combinations. From/Into are not necessarily a good match especially for lossy conversions like Color3f->Color3.