Update dependencies and fix new warnings

Notable changes
* Updating Embassy was, as usual, a minor pain.
  * Embassy requires a more recent of the nightly compiler.
  * There was a breaking API change to a embassy_sync channel API, but
    luckily it was just a name change.
* The compiler advised against using async functions in public traits.
Rather than following the recommended syntax workaround, I took the hint
and reduced the visibility of the SampleProducer traits, which don't
need to be public.
* Remove the fix-hidden-lifetime-bug crate, which is no longer needed as
  of Rust 1.69.