my.core.serialize: implement default flag

Lots of surrounding changes from this discussion:
0593c69056

renamed/split error_to_json into error_to_row, moved
pandas-aware functions to my.core.pandas, leaving
generic ones in my.core.error.

tox.ini

@@ -23,6 +23,7 @@ setenv = MY_CONFIG = nonexistent
 commands =
     pip install -e .[testing]
     pip install cachew
+    pip install orjson
 
     hpi module install my.location.google
     pip install ijson # optional dependency