Rename `.msg.types.Msg` -> `PayloadMsg`

tractor/msg/types.py

@@ -56,8 +56,7 @@ log = get_logger('tractor.msgspec')
 PayloadT = TypeVar('PayloadT')
 
 
-# TODO: PayloadMsg
-class Msg(
+class PayloadMsg(
     Struct,
     Generic[PayloadT],
 
@@ -110,6 +109,10 @@ class Msg(
     pld: Raw
 
 
+# TODO: complete rename
+Msg = PayloadMsg
+
+
 class Aid(
     Struct,
     tag=True,