py: Allow bytecode/native to put iter_buf on stack for simple for loops.

So that the "for x in it: ..." statement can now work without using the
heap (so long as the iterator argument fits in an iter_buf structure).
diff --git a/py/bc0.h b/py/bc0.h
index 5ff9e50..c2b019f 100644
--- a/py/bc0.h
+++ b/py/bc0.h
@@ -79,6 +79,7 @@
 #define MP_BC_POP_BLOCK          (0x44)
 #define MP_BC_POP_EXCEPT         (0x45)
 #define MP_BC_UNWIND_JUMP        (0x46) // rel byte code offset, 16-bit signed, in excess; then a byte
+#define MP_BC_GET_ITER_STACK     (0x47)
 
 #define MP_BC_BUILD_TUPLE        (0x50) // uint
 #define MP_BC_BUILD_LIST         (0x51) // uint