set* versions of color channel functions

[matthewwithanm]

Currently, most of the color operation functions modify a channel relative to its current value. (fade is the exception here, allowing you to specify the alpha value to use for the result, regardless of the input value.)

It would be nice if there were set* functions for each of the color channels. For example, setlightness(@c, @lightness) would be equivalent to hsl(hue(@c), saturation(@c), @lightness) in the same way that fade(@c, @alpha) is to rgba(red(@c), green(@c), blue(@c), @alpha).

[seven-phases-max]

Implement as a plugin (first). See #1853 for more on the current "new color functions" approach.