nac3artiq: add simple KernelInvariant CPython wrapper

nac3artiq/min_artiq.py

@@ -8,7 +8,7 @@ import nac3artiq
 import device_db
 
 
-__all__ = ["extern", "kernel", "portable", "Core", "TTLOut"]
+__all__ = ["KernelInvariant", "extern", "kernel", "portable", "Core", "TTLOut"]
 
 
 nac3 = nac3artiq.NAC3(device_db.device_db["core"]["arguments"]["target"])
@@ -16,6 +16,10 @@ allow_module_registration = True
 registered_ids = set()
 
 
+def KernelInvariant(t):
+    return t
+
+
 def register_module_of(obj):
     global registered_ids
     assert allow_module_registration