rlp: list length and contents are not validated

[divagant-martian]

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