rlp: list length and contents are not validated

Three examples that should be rejected

    let invalid_rlp = [193, 4, 4];
    let rlp = rlp::Rlp::new(&invalid_rlp);
    let item_count = rlp.item_count().unwrap(); // this should fail
    assert_eq!(item_count, 2); // wrong. invalid rlp with a list of one element followed by extra
                               // data that ends up being assumed as part of the list

    let invalid_rlp = [195, 2, 247, 247];
    let rlp = rlp::Rlp::new(&invalid_rlp);
    let item_count = rlp.item_count().unwrap(); // should fail
    assert_eq!(item_count, 1); // wrong. invalid rlp with a list of 3 bytes where only the first
                               // one is valid rlp

    let invalid_rlp = [194, 1];
    let rlp = rlp::Rlp::new(&invalid_rlp);
    let item_count = rlp.item_count().unwrap(); // should fail
    assert_eq!(item_count, 0); // wrong. list should occupy two bytes, occupies just one and it's
                               // still accepted

Start at the Rlp::item_count entry point and trace how list lengths and nested item boundaries are validated. Add regression coverage for the three byte sequences in the issue, with completion meaning each malformed list is rejected rather than returning an item count.