bpo-43683: Handle generator entry in bytecode (GH-25138)

* Handle check for sending None to starting generator and coroutine into bytecode.

* Document new bytecode and make it fail gracefully if mis-compiled.
diff --git a/Python/opcode_targets.h b/Python/opcode_targets.h
index 692442a..e20a898 100644
--- a/Python/opcode_targets.h
+++ b/Python/opcode_targets.h
@@ -128,7 +128,7 @@
     &&TARGET_DELETE_FAST,
     &&_unknown_opcode,
     &&_unknown_opcode,
-    &&_unknown_opcode,
+    &&TARGET_GEN_START,
     &&TARGET_RAISE_VARARGS,
     &&TARGET_CALL_FUNCTION,
     &&TARGET_MAKE_FUNCTION,