Support negative row offsets

[recmo]

On 2020-05-06 @pvienhage wrote in af4b6e2 “Basic autogeneration (#687)”:

Support negative row offsets

        Ok(file) => file,
    };
    let _ = writeln!(&mut file, "{}", trace_contract);
}

// TODO - Support negative row offsets
fn autogen_trace_layout(
    trace_keys: &[&RationalExpression],
    n_cols: usize,
    constraint_degree: usize,
    blowup: usize,

From crypto/stark/src/solidity_verifier.rs:286