v2020-06-20

Simplify the interface between elm app and web host

+ Remove redundant requests to be notified on the arrival of time.
+ Clarify there is no dependency between the different tasks, as outlined at https://github.com/elm-fullstack/elm-fullstack/blob/6fa473cd9a6287e1ef452a1d736d445330e79739/explore/2019-08-31.interface-between-process-and-host/2019-08-31.interface-between-process-and-host.md
+ For now, continue supporting the old interface. Implement a branch in the host to use the version of the interface matching the Elm app code.
+ Adapt enough examples in tests to cover the different messages on the new interface.
+ Update the names of JSON fields on the interface to use the consistent casing of the first letters.
+ Also in the new version of the interface Elm module: Adapt to recent observations of conversions between `Bytes.Bytes` and other representations: Use base64 for now because the conversions cost a lot of time with the current execution engine. As can be seen in some examples, we need to convert back to base64 again anyway.