Add `combine` functions to structures These combine two structures by appending the lists in each, and using a passed record to determine how to combine non-list fields.
| Time to Start | Worker time | Duration | Time to finish | |
| Config | 1s | 1s | 1s | 3s |
| Eval | 3s | 6s | 6s | 10s |
| Build | 11s | 1m13s | 1m17s | 1m29s |
| Suite | 1s | 1m21s | 1m27s | 1m29s |