Export color constants as associated constants on Color

[noeddl]

Fixes #171.

Some notes:

• As we discussed in the issue, I replaced the static variables COLORS and COLOR_NAMES with the functions all_colors and all_color_names, respectively. If we keep the extended (now extra_colors module) we should probably do the same thing there so that it's consistent? (In any case, the documentation should be updated to refer to the functions instead of the static variables.)
• The generated documentation for associated constants is not as compact as the one for module-level constants. I think we could maybe remove the line #[doc = $name] in color_consts! as this does not provide any more information about the respective constant (other than repeating the name of the color it refers to). This would safe some of the space that the constants take up in the docs.

[noeddl]

The extra #[doc = $name] is important because it tells the user what to use when referring to the color in a string literal. For example, the color CHERRY_RED needs to be referred to as "cherry red" when calling set_pen_color. I'm planning to rework things so this is more clear.

I see, that makes sense.

Thanks for merging!