rustdoc: unions and consts are not color-coded

[QuietMisdreavus]

All the various kinds of item in rust are given a color in rustdoc's CSS, but consts and unions never got one. Fixing should just be a matter of collecting the class name that these items use, and adding a couple rules to rustdoc.css to apply some color to them. The question of which color is another matter entirely. >_>

[GuillaumeGomez]

I'll propose something out. :)