Colorize core, border and crown in that order

Property definitions

nitc $ POSetGroupColorer :: compute_colors
	# Colorize core, border and crown in that order
	private fun compute_colors do
		colors_cache.clear
		colorize_core
		colorize_set(graph.border)
		colorize_set(graph.crown)
	end
src/compiler/coloring.nit:351,2--357,4