WIP: feat: better "last updated" function; compute the total time in this state

[grunweg]

Note: carefully check if this is the right version of the branch before merging, or if the version not from a fork is better.

This PR implements the "algorithm" half of #7 and #33: given the right input data, this computes

• the total time this PR was e.g. waiting for review
• the current time this PR is in that state

The main ingredients are well-tested; the overall algorithm could be tested a bit more. Steps before this can be merged

• [ ] agree if the overall approach looks sensible
• [ ] clean up the debugging output; add tests for the last bug found

Best reviewed as the whole diff.