Collect all init methods introduced in self

Property definitions

nitc :: model_collect $ MClass :: collect_intro_inits
	# Collect all init methods introduced in `self`
	fun collect_intro_inits(filter: nullable ModelFilter): Set[MMethod] do
		var res = new HashSet[MMethod]
		for mproperty in collect_intro_mmethods(filter) do
			if mproperty.is_init then res.add(mproperty)
		end
		return res
	end
src/model/model_collect.nit:875,2--882,4