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.
This patch removes unused crate imports reported by newer rust versions.
Some crates were only required for tests, some only for tests with
macro_import - these things were fixed with feature gates.
