Drop `Channel.aiter_recv()`

Internalize the implementation of this and expect client code to
iterate the `Channel` directly.

Resolves #16

tractor/_actor.py

@@ -278,7 +278,7 @@ class Actor:
         # worked out we'll likely want to use that!
         log.debug(f"Entering msg loop for {chan} from {chan.uid}")
         try:
-            async for msg in chan.aiter_recv():
+            async for msg in chan:
                 if msg is None:  # terminate sentinel
                     log.debug(
                         f"Cancelling all tasks for {chan} from {chan.uid}")

tractor/_ipc.py

@@ -87,6 +87,7 @@ class Channel:
         self._destaddr = destaddr or self.squeue.raddr
         # set after handshake - always uid of far end
         self.uid = None
+        self._agen = self._aiter_recv()
 
     def __repr__(self):
         if self.squeue:
@@ -134,8 +135,8 @@ class Channel:
     async def __aexit__(self, *args):
         await self.aclose(*args)
 
-    async def __aiter__(self):
-        return self.aiter_recv()
+    def __aiter__(self):
+        return self._agen
 
     async def _reconnect(self):
         """Handle connection failures by polling until a reconnect can be
@@ -166,7 +167,7 @@ class Channel:
                         " for re-establishment")
                 await trio.sleep(1)
 
-    async def aiter_recv(self):
+    async def _aiter_recv(self):
         """Async iterate items from underlying stream.
         """
         while True: