json trace: log error on non representable arguments

whenever an event contains an argument which cannot be represented
in json, we log a special entry with:
  cat="spdr-error"
  name="arg-serialization"

and the string representation of the first argument that could not be
represented
Merge branch 'F-NaNorInf'

Handle NaN and Inf and produce 0.0 rather than null.

The error is logged as a dedicated error event in the stream
for easy inspection.

closing: https://github.com/uucidl/uu.spdr/issues/6

* F-NaNorInf:
  json trace: log error on non representable arguments