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

Property definitions

core $ BytePattern :: first_index_in
	# Return the first occurence of `self` in `b`, or -1 if not found
	fun first_index_in(b: SequenceRead[Int]): Int do return first_index_in_from(b, 0)
lib/core/bytes.nit:24,2--25,82