my.core.serialize: orjson with additional default and _serialize hook (#140)

basic orjson serialize, json.dumps fallback

Lots of surrounding changes from this discussion:
0593c69056

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