Collect all methods definitions in self

Property definitions

nitc :: model_collect $ MClassDef :: collect_mmethoddefs
	# Collect all methods definitions in `self`
	fun collect_mmethoddefs(filter: nullable ModelFilter): Set[MMethodDef] do
		var res = new HashSet[MMethodDef]
		for mpropdef in collect_mpropdefs(filter) do
			if not mpropdef isa MMethodDef then continue
			res.add mpropdef
		end
		return res
	end
src/model/model_collect.nit:1033,2--1041,4