- Tracing is very verbose, to the point of not being useful, and it
    is hard to remove personal information from it.  Make full tracing
    opt-in even for development versions.

  - Introduce SEQUOIA_OCTOPUS_TRACING to control tracing.  Available
    modes are:

    - By default, only failures are traced.

    - "call" traces all function invocations with their arguments and
      results.

    - "full" selects the previous tracing mode.

  - Introduce a new type `RnpStatus` that wraps `RnpResult` for the RNP
    status code constants.  By using a type distinct from `RnpResult`,
    we force a conversion through `RnpStatus::epilogue` (or
    `RnpStatus::quiet_epilogue`), and this is the point where we can
    emit log messages on failures.