ast: Use better parameter serialization for paramod names.

ast: Use better parameter serialization for paramod names.

Calling log_signal is problematic for several reasons:

- with recent changes, empty string is serialized as { }, which violates
  the "no spaces in IdString" rule
- the type (plain / real / signed / string) is dropped, wrongly conflating
  functionally different values and potentially introducing a subtle
  elaboration bug

Instead, use a custom simple serialization scheme.