Move tractor._shm to tractor.ipc._shm

EventFD class now expects the fd to already be init with open_eventfd
RingBuff Sender and Receiver fully manage SharedMemory and EventFD lifecycles, no aditional ctx mngrs needed
Separate ring buf tests into its own test bed
Add parametrization to test and cancellation
Add docstrings
Add simple testing data gen module .samples

tests/test_ringbuf.py

@@ -37,10 +37,6 @@ async def child_read_shm(
             msg = await receiver.receive_some()
             recvd_bytes += len(msg)
 
-        # make sure we dont hold any memoryviews
-        # before the ctx manager aclose()
-        msg = None
-
     end_ts = time.time()
     elapsed = end_ts - start_ts
     elapsed_ms = int(elapsed * 1000)