Improve TypeObject.toString()

For TypeObject we end up emitting toString() in the form of:

PlspId{_value=0}

This does not quite align what we do for DataObjects, as there we use "value" instead of "_value".

Note TypeObjects include also keys and other constructs, so they legally can have differently-named properties.

At the very least we need to use a proper property name (not field name). Perhaps we could even omit the property name in case of single-attribute TypeObjects?