compiler: allows string annotation

According to PEP484, type hint can be a string literal for forward
references. With PEP563, type hint would be preserved in annotations in
string form.

artiq/compiler/embedding.py

@@ -949,6 +949,27 @@ class Stitcher:
         if annot is None:
             annot = builtins.TNone()
 
+        if isinstance(function, SpecializedFunction):
+            host_function = function.host_function
+        else:
+            host_function = function
+
+        embedded_function = host_function.artiq_embedded.function
+
+        if isinstance(embedded_function, str):
+            embedded_function = host_function
+
+        if isinstance(annot, str):
+            if annot not in embedded_function.__globals__:
+                diag = diagnostic.Diagnostic(
+                        "error",
+                        "type annotation for {kind}, {annot}, is not defined",
+                        {"kind": kind, "annot": repr(annot)},
+                        self._function_loc(function),
+                        notes=self._call_site_note(call_loc, fn_kind))
+                self.engine.process(diag)
+            annot = embedded_function.__globals__[annot]
+
         if not isinstance(annot, types.Type):
             diag = diagnostic.Diagnostic("error",
                 "type annotation for {kind}, '{annot}', is not an ARTIQ type",