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/Lib/opcode.py b/Lib/opcode.py
index b119712..196e6f8 100644
--- a/Lib/opcode.py
+++ b/Lib/opcode.py
@@ -173,6 +173,7 @@ def jabs_op(name, op):
 def_op('DELETE_FAST', 126)      # Local variable number
 haslocal.append(126)
 
+def_op('GEN_START', 129)        # Kind of generator/coroutine
 def_op('RAISE_VARARGS', 130)    # Number of raise arguments (1, 2, or 3)
 def_op('CALL_FUNCTION', 131)    # #args
 def_op('MAKE_FUNCTION', 132)    # Flags