Appends ln bytes from ns starting at index from to self

Property definitions

core $ Bytes :: append_ns_from
	# Appends `ln` bytes from `ns` starting at index `from` to self
	fun append_ns_from(ns: CString, ln, from: Int) do
		if persisted then regen
		var nlen = length + ln
		if nlen > capacity then enlarge(nlen)
		ns.copy_to(items, ln, from, length)
		length += ln
	end
lib/core/bytes.nit:586,2--593,4