Re-think, `msgspec`-multi-typed msg dialogs

The greasy details are strewn throughout a `msgspec` issue:
https://github.com/jcrist/msgspec/issues/140

and specifically this code was mostly written as part of POC example in
this comment:
https://github.com/jcrist/msgspec/issues/140#issuecomment-1177850792

This work obviously pertains to our desire and prep for typed messaging
and capabilities aware msg-oriented-protocols in #196. I added a "wants
to have" method to `Context` showing how I think we could offer a pretty
neat msg-type-set-as-capability-for-protocol system.

XXX NOTE XXX: this commit was rewritten during a rebase from a very old
version as per the prior commit.

tractor/_streaming.py

@@ -533,6 +533,17 @@ class MsgStream(trio.abc.Channel):
             else:
                 raise
 
+    # TODO: msg capability context api1
+    # @acm
+    # async def enable_msg_caps(
+    #     self,
+    #     msg_subtypes: Union[
+    #         list[list[Struct]],
+    #         Protocol,   # hypothetical type that wraps a msg set
+    #     ],
+    # ) -> tuple[Callable, Callable]:  # payload enc, dec pair
+    #     ...
+
 
 def stream(func: Callable) -> Callable:
     '''

tractor/msg/_old_msg.py

@@ -48,11 +48,12 @@ and APIs for applying custom msgspec-sets for implementing un-protocol state mac
 from __future__ import annotations
 from contextlib import contextmanager as cm
 from typing import (
-    Union,
     Any,
+    Optional,
+    Union,
 )
 
-from msgspec import Struct
+from msgspec import Struct, Raw
 from msgspec.msgpack import (
     Encoder,
     Decoder,
@@ -95,8 +96,6 @@ def configure_native_msgs(
     in all IPC transports and pop the codec on exit.
 
     '''
-    global _lifo_codecs
-
     # See "tagged unions" docs:
     # https://jcristharif.com/msgspec/structs.html#tagged-unions
 
@@ -119,3 +118,91 @@ def configure_native_msgs(
     finally:
         print("NONONONONON")
         _lifo_codecs.pop()
+
+
+class Header(Struct, tag=True):
+    '''
+    A msg header which defines payload properties
+
+    '''
+    uid: str
+    msgtype: Optional[str] = None
+
+
+class Msg(Struct, tag=True):
+    '''
+    The "god" msg type, a box for task level msg types.
+
+    '''
+    header: Header
+    payload: Raw
+
+
+_root_dec = Decoder(Msg)
+_root_enc = Encoder()
+
+# sub-decoders for retreiving embedded
+# payload data and decoding to a sender
+# side defined (struct) type.
+_subdecs:  dict[
+    Optional[str],
+    Decoder] = {
+    None: Decoder(Any),
+}
+
+
+@cm
+def enable_context(
+    msg_subtypes: list[list[Struct]]
+) -> Decoder:
+
+    for types in msg_subtypes:
+        first = types[0]
+
+        # register using the default tag_field of "type"
+        # which seems to map to the class "name".
+        tags = [first.__name__]
+
+        # create a tagged union decoder for this type set
+        type_union = Union[first]
+        for typ in types[1:]:
+            type_union |= typ
+            tags.append(typ.__name__)
+
+        dec = Decoder(type_union)
+
+        # register all tags for this union sub-decoder
+        for tag in tags:
+            _subdecs[tag] = dec
+        try:
+            yield dec
+        finally:
+            for tag in tags:
+                _subdecs.pop(tag)
+
+
+def decmsg(msg: Msg) -> Any:
+    msg = _root_dec.decode(msg)
+    tag_field = msg.header.msgtype
+    dec = _subdecs[tag_field]
+    return dec.decode(msg.payload)
+
+
+def encmsg(
+    dialog_id: str | int,
+    payload: Any,
+) -> Msg:
+
+    tag_field = None
+
+    plbytes = _root_enc.encode(payload)
+    if b'type' in plbytes:
+        assert isinstance(payload, Struct)
+        tag_field = type(payload).__name__
+        payload = Raw(plbytes)
+
+    msg = Msg(
+        Header(dialog_id, tag_field),
+        payload,
+    )
+    return _root_enc.encode(msg)