Consider optionally allowing named segments as evaluator parameters

Now that we have made evaluators more general in terms of segments it can accept, it may be helpful to optionally allow named segments in parameters. For example, consider a trace with 5 segments (`segment_0`, `segment_1`, `segment_2`, `segment_3`, `segment_4`) and an evaluator that takes first and last segment.

Something like `ev evaluator([a, b], [], [], [], [c, d])` is probably prone to error. We should also allow users to do something like:
`ev evaluator(segment_0: [a, b], segment_4: [c, d])`.

This issue is not of high priority though.