Fix returning tuples of lists of arrays from RPCs

When serialising a list of objects `_send_rpc_value` makes a copy of the
upcoming tags to pass repeatedly to the recursive call. Then uses
`_skip_rpc_value` to skip over the tags that should have been processed.
This didn't handle numpy arrays so, after processing a list of arrays it
got out of sync and failed.

Signed-off-by: Michael Birtwell <michael.birtwell@oxionics.com>

artiq/coredevice/comm_kernel.py

@@ -418,6 +418,9 @@ class CommKernel:
             self._skip_rpc_value(tags)
         elif tag == "r":
             self._skip_rpc_value(tags)
+        elif tag == "a":
+            _ndims = tags.pop(0)
+            self._skip_rpc_value(tags)
         else:
             pass