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.

setup.py

@@ -55,6 +55,7 @@ def main():
             'optional': [
                 # todo document these?
                 'logzero',
+                'orjson',
                 'cachew>=0.8.0',
                 'mypy', # used for config checks
             ],