Replace open-coded Debug impls with educe or custom_debug

!360 (merged) and !353 (merged) have formulaic open-coded Debug impls. Probably, we will want more like this. We should replace these with the use of some macro library.

Suggestions so far are educe and custom_debug (!360 (comment 2781864)).

I think I prefer educe because it can do other things too (eg, Clone as needed in !355 (d5685461) due to wrong automatic bounds from std's Clone macro) - and it seems more popular going by download stats at least. But it does lack that format!()-style convenience syntax for custom debugs.