Re-use interpolation function form TraceTable

[recmo]

On 2020-04-04 @recmo wrote in 6637600 “Add timing spans”:

Re-use interpolation function form TraceTable

            }
        });
    trace!("END Evaluate");

    info!("Convert from values to coefficients");
    // TODO: Re-use interpolation function form TraceTable
    trace!("BEGIN Interpolate");
    values.ifft();
    permute(values);
    trace!("END Interpolate");
    // OPT: Merge with even-odd separation loop.

From crypto/stark/src/prover.rs:602