Also change that the implementation uses toml_query::set instead of
toml_query::insert.
Inserting values creates intermediate tables, set doesn't. And we really
want that convenience here, as the code is complex enough on its own.
This merge solved a _LOT_ of conflicts and was a rather complicated one,
as parts of the conflict-resolution involved rewriting of half the
stuff.
This merge commit fixes all the things so a `cargo check --all`
succeeds, but I did not yet check whether tests run without failure.
