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.

tests/core.py

@@ -18,3 +18,4 @@ from my.core.util           import *
 from my.core.discovery_pure import *
 from my.core.types          import *
 from my.core.stats          import *
+from my.core.serialize      import *