WIP tagged union message type API

XXX NOTE XXX: this is a heavily modified commit from the original
(ec226463) which was super out of date when rebased onto the current
branch. I went through a manual conflict rework and removed all the
legacy segments as well as rename-moved this original mod
`tractor.msg.py` -> `tractor.msg/_old_msg.py`. Further the
`NamespacePath` type def was discarded from this mod since it was from
a super old version which was already moved to a `.msg.ptr` submod.

As per original questions and discussion with `msgspec` author:
- https://github.com/jcrist/msgspec/issues/25
- https://github.com/jcrist/msgspec/issues/140

this prototypes a new (but very naive) `msgspec.Struct` codec
implementation which will be more filled out in the next commit.

tractor/_ipc.py

@@ -165,7 +165,9 @@ class MsgpackTCPStream(MsgTransport):
         self.codec: MsgCodec = codec or MsgCodec()
 
     async def _iter_packets(self) -> AsyncGenerator[dict, None]:
-        '''Yield packets from the underlying stream.
+        '''
+        Yield `bytes`-blob decoded packets from the underlying TCP
+        stream using the current task's `MsgCodec`.
 
         '''
         import msgspec  # noqa

tractor/msg/_old_msg.py

@@ -0,0 +1,121 @@
+# tractor: structured concurrent "actors".
+# Copyright 2018-eternity Tyler Goodlet.
+
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Affero General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU Affero General Public License for more details.
+
+# You should have received a copy of the GNU Affero General Public License
+# along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+'''
+Capability-based messaging specifications: or colloquially as "msgspecs".
+
+Includes our SCIPP (structured-con-inter-process-protocol) message type defs
+and APIs for applying custom msgspec-sets for implementing un-protocol state machines.
+
+'''
+
+# TODO: integration with our ``enable_modules: list[str]`` caps sys.
+
+# ``pkgutil.resolve_name()`` internally uses
+# ``importlib.import_module()`` which can be filtered by inserting
+# a ``MetaPathFinder`` into ``sys.meta_path`` (which we could do before
+# entering the ``Actor._process_messages()`` loop).
+# https://github.com/python/cpython/blob/main/Lib/pkgutil.py#L645
+# https://stackoverflow.com/questions/1350466/preventing-python-code-from-importing-certain-modules
+#   - https://stackoverflow.com/a/63320902
+#   - https://docs.python.org/3/library/sys.html#sys.meta_path
+
+# the new "Implicit Namespace Packages" might be relevant?
+# - https://www.python.org/dev/peps/pep-0420/
+
+# add implicit serialized message type support so that paths can be
+# handed directly to IPC primitives such as streams and `Portal.run()`
+# calls:
+# - via ``msgspec``:
+#   - https://jcristharif.com/msgspec/api.html#struct
+#   - https://jcristharif.com/msgspec/extending.html
+# via ``msgpack-python``:
+# https://github.com/msgpack/msgpack-python#packingunpacking-of-custom-data-type
+
+from __future__ import annotations
+from contextlib import contextmanager as cm
+from typing import (
+    Union,
+    Any,
+)
+
+from msgspec import Struct
+from msgspec.msgpack import (
+    Encoder,
+    Decoder,
+)
+
+
+# LIFO codec stack that is appended when the user opens the
+# ``configure_native_msgs()`` cm below to configure a new codec set
+# which will be applied to all new (msgspec relevant) IPC transports
+# that are spawned **after** the configure call is made.
+_lifo_codecs: list[
+    tuple[
+        Encoder,
+        Decoder,
+    ],
+] = [(Encoder(), Decoder())]
+
+
+def get_msg_codecs() -> tuple[
+    Encoder,
+    Decoder,
+]:
+    '''
+    Return the currently configured ``msgspec`` codec set.
+
+    The defaults are defined above.
+
+    '''
+    global _lifo_codecs
+    return _lifo_codecs[-1]
+
+
+@cm
+def configure_native_msgs(
+    tagged_structs: list[Struct],
+):
+    '''
+    Push a codec set that will natively decode
+    tagged structs provied in ``tagged_structs``
+    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
+
+    # "The quickest way to enable tagged unions is to set tag=True when
+    # defining every struct type in the union. In this case tag_field
+    # defaults to "type", and tag defaults to the struct class name
+    # (e.g. "Get")."
+    enc = Encoder()
+
+    types_union = Union[tagged_structs[0]] | Any
+    for struct in tagged_structs[1:]:
+        types_union |= struct
+
+    dec = Decoder(types_union)
+
+    _lifo_codecs.append((enc, dec))
+    try:
+        print("YOYOYOOYOYOYOY")
+        yield enc, dec
+    finally:
+        print("NONONONONON")
+        _lifo_codecs.pop()