[CX] trace function

Add a trace function to simplify analysis and debugging of queries.
The function takes 2 arguments:

• an arbitrary instance inst of the data model and
• a string label label.

When the function is evaluated

1. inst is returned and
2. a serialized version of inst labelled with label is written to a trace channel.

The content of the trace channel can either

• be returned with the result or
• written to temporary storage and retrieved at a later point in time.