The introduction of mclass_c

Property definitions

nitc $ ModelDiamond :: mclassdef_c=
	# The introduction of `mclass_c`
	var mclassdef_c: MClassDef do
		var res = new MClassDef(mmodule0, mclass_c.mclass_type, location)
		res.set_supertypes([mclass_a.mclass_type])
		res.add_in_hierarchy
		return res
	end
src/model/model.nit:2827,2--2833,4