Return the last occurence of self in b, or -1 if not found

Property definitions

core $ BytePattern :: last_index_in
	# Return the last occurence of `self` in `b`, or -1 if not found
	fun last_index_in(b: SequenceRead[Int]): Int do return last_index_in_from(b, b.length - 1)
lib/core/bytes.nit:30,2--31,91