`or` is not accepted as an identifier

[darichey]

Following #106 and #116, or is no longer accepted as an identifier.

fn main() {
    let nix_expr = "{ or = 1; }";
    println!(
        "{:#?}",
        rnix::Root::parse(&nix_expr).ok().unwrap()
    );
}

thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: UnexpectedWanted(TOKEN_OR, 2..11, [TOKEN_IDENT])', src/main.rs:5:43