firmware: Fix object references in tuples

Sine 8740ec3dd, the alignment() information from
"run-time type information" (i.e. the Tag type) is also
used when sending tuples to the host.

artiq/firmware/libproto_artiq/rpc_proto.rs

@@ -429,8 +429,8 @@ mod tag {
                 // the ptr/length(s) pair is basically CSlice
                 Tag::Bytes | Tag::String | Tag::ByteArray | Tag::List(_) | Tag::Array(_, _) =>
                     core::mem::align_of::<CSlice<()>>(),
-                // will not be sent from the host
-                _ => unreachable!("unexpected tag from host")
+                Tag::Keyword(_) => unreachable!("Tag::Keyword should not appear in composite types"),
+                Tag::Object => core::mem::align_of::<u32>(),
             }
         }
 

artiq/test/coredevice/test_embedding.py

@@ -74,6 +74,9 @@ class RoundtripTest(ExperimentCase):
     def test_object_list(self):
         self.assertRoundtrip([object(), object()])
 
+    def test_object_tuple(self):
+        self.assertRoundtrip((False, object(), True, 0x12345678))
+
     def test_list_tuple(self):
         self.assertRoundtrip(([1, 2], [3, 4]))