Try out `msgspec` encode-buffer optimization

As per the reco:
https://jcristharif.com/msgspec/perf-tips.html#reusing-an-output-buffe

BUT, seems to cause this error in `pikerd`..

`BufferError: Existing exports of data: object cannot be re-sized`

Soo no idea? Maybe there's a tweak needed that we can glean from
tests/examples in the `msgspec` repo?

Disabling for now.

tractor/msg/__init__.py

@@ -37,6 +37,11 @@ from ._codec import (
     MsgDec as MsgDec,
     current_codec as current_codec,
 )
+# currently can't bc circular with `._context`
+# from ._ops import (
+#     PldRx as PldRx,
+#     _drain_to_final_msg as _drain_to_final_msg,
+# )
 
 from .types import (
     Msg as Msg,

tractor/msg/_codec.py

@@ -280,16 +280,31 @@ class MsgCodec(Struct):
     def enc(self) -> msgpack.Encoder:
         return self._enc
 
+    # TODO: reusing encode buffer for perf?
+    # https://jcristharif.com/msgspec/perf-tips.html#reusing-an-output-buffer
+    _buf: bytearray = bytearray()
+
     def encode(
         self,
         py_obj: Any,
 
+        use_buf: bool = False,
+        # ^-XXX-^ uhh why am i getting this?
+        # |_BufferError: Existing exports of data: object cannot be re-sized
+
     ) -> bytes:
         '''
         Encode input python objects to `msgpack` bytes for
         transfer on a tranport protocol connection.
 
+        When `use_buf == True` use the output buffer optimization:
+        https://jcristharif.com/msgspec/perf-tips.html#reusing-an-output-buffer
+
         '''
+        if use_buf:
+            self._enc.encode_into(py_obj, self._buf)
+            return self._buf
+        else:
             return self._enc.encode(py_obj)
 
     @property