def consume_item(rlp, start)
      t, l, s = consume_length_prefix(rlp, start)
      consume_payload(rlp, s, t, l)
    end
def consume_item(rlp, start)
      t, l, s = consume_length_prefix(rlp, start)
      consume_payload(rlp, s, t, l)
    end
Read an item from an RLP string.

 * `rlp` - the string to read from
 * `start` - the position at which to start reading`

 Returns a pair `[item, end]` where `item` is the read item and `end` is
 the position of the first unprocessed byte.