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

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 test_serialize_fallback

tests/serialize.py

@@ -0,0 +1 @@
+from my.core.serialize      import *