compiler: Allow `None` in type hints

Similar to how Python itself interprets None as type(None),
make it translate to TNone in ARTIQ compiler type hints.

artiq/compiler/embedding.py

@@ -950,6 +950,9 @@ class Stitcher:
         return function_node
 
     def _extract_annot(self, function, annot, kind, call_loc, fn_kind):
+        if annot is None:
+            annot = builtins.TNone()
+
         if not isinstance(annot, types.Type):
             diag = diagnostic.Diagnostic("error",
                 "type annotation for {kind}, '{annot}', is not an ARTIQ type",

artiq/test/lit/embedding/annotation.py

@@ -11,7 +11,12 @@ def foo(x: TInt64, y: TInt64 = 1) -> TInt64:
     print(x+y)
     return x+y
 
+@kernel
+def bar(x: TInt64) -> None:
+    print(x)
+
 @kernel
 def entrypoint():
     print(foo(0, 2))
     print(foo(1, 3))
+    bar(3)

artiq/test/lit/embedding/error_specialized_annot.py

@@ -4,9 +4,9 @@
 from artiq.experiment import *
 
 class c():
-# CHECK-L: ${LINE:+2}: error: type annotation for argument 'x', 'None', is not an ARTIQ type
+# CHECK-L: ${LINE:+2}: error: type annotation for argument 'x', '<class 'float'>', is not an ARTIQ type
     @kernel
-    def hello(self, x: None):
+    def hello(self, x: float):
         pass
 
     @kernel