Make TODO comments more specific and prune them

[kohr-h]

During my repr improvement work, I've also gone though a bunch of TODO comments that just made me scratch my head. Stuff like TODO: update docs... okay, how? (That one was by me btw).

So I'd propose that we be more specific when writing comments like this, and make such comments easier to search. Here's my suggestion:

• Write # TODO(<username>): ... for generic comments on implementation details, suboptimal code, reasons for not implementing things etc. This way it's at least clear whom to ask what the comment was about.
• Write # TODO(#<issue number>): ... for TODOs that have an issue attached to them, for instance "do this when something gets available/fixed".
• Update # TODO(#<issue number>): ... -> # TODO(doable, #<issue number>): ... when the issue has been closed in the meanwhile. This update should be done for every issue that gets closed, a simple text search brings up the candidates (see above).
• Write # TODO(PR #<PR number>): ... for something that will be addressed by a PR that's underway.
• For upstream issues, write # TODO(<package> #...) with PR or issue as above.