Add a `Context` type for task/protocol RPC state

This is loosely based off the nanomsg concept found here:
https://nanomsg.github.io/nng/man/v1.1.0/nng_ctx.5

tractor/__init__.py

@@ -11,7 +11,7 @@ import trio  # type: ignore
 from trio import MultiError
 
 from .log import get_console_log, get_logger, get_loglevel
-from ._ipc import _connect_chan, Channel
+from ._ipc import _connect_chan, Channel, Context
 from ._actor import (
     Actor, _start_actor, Arbiter, get_arbiter, find_actor, wait_for_actor
 )

tractor/_ipc.py

@@ -205,6 +205,28 @@ class Channel:
         return self.squeue.connected() if self.squeue else False
 
 
+class Context:
+    """An IAC (inter-actor communication) context.
+
+    Allows maintaining task or protocol specific state between communicating
+    actors. A unique context is created on the receiving end for every request
+    to a remote actor.
+    """
+    def __init__(
+        self,
+        channel: Channel,
+        command_id: str,
+    ):
+        self.chan: Channel = channel
+        self.cid: str = command_id
+
+    async def send_yield(self, data):
+        await self.chan.send({'yield': data, 'cid': self.cid})
+
+    async def send_stop(self):
+        await self.chan.send({'stop': True, 'cid': self.cid})
+
+
 @asynccontextmanager
 async def _connect_chan(
     host: str, port: int