Collect all attributes redefined in self

Property definitions

nitc :: model_collect $ MClass :: collect_redef_mattributes
	# Collect all attributes redefined in `self`
	fun collect_redef_mattributes(filter: nullable ModelFilter): Set[MAttribute] do
		var res = new HashSet[MAttribute]
		for mproperty in collect_redef_mproperties(filter) do
			if mproperty isa MAttribute then res.add(mproperty)
		end
		return res
	end
src/model/model_collect.nit:838,2--845,4