Should check the last byte in Id::from_str

[zensh]

A bad case:

let s = "00000000000000jarvis";
assert_eq!(Id::from_str(s).unwrap().to_string().as_str(), s);

thread 'id::tests::test_from_str_should_check' panicked at 'assertion failed: (left == right)
left: "00000000000000jarvig",
right: "00000000000000jarvis"'

https://github.com/rs/xid/blob/master/id.go#L267

[kazk]

Sorry, I just noticed this. So the original issue is rs/xid#71, and we need to reject invalid strings not generated by xid, which can be done efficiently by checking the last byte.
Thanks, I'll review the PR soon.