Panic on input that is not valid UTF8

[dtolnay]

This should be an error not a panic.

fn main() {
    let data = &[b'"', b'\xed', b'\xa0', b'\x80', b'"'];
    serde_json::from_slice::<String>(data).ok();
}

thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: Utf8Error { valid_up_to: 0 }'

Caused by this unwrap.

[oli-obk]

we could generally enable the clippy lint for denying unwrap

[dtolnay]

I fixed the panic in #100 and I filed #101 to consider enabling the Clippy lint.