py/obj: Merge getiter and iternext mp_obj_type_t slots.

The goal here is to remove a slot (making way to turn make_new into a slot)
as well as reduce code size by the ~40 references to mp_identity_getiter
and mp_stream_unbuffered_iter.

This introduces two new type flags:
- MP_TYPE_FLAG_ITER_IS_ITERNEXT: This means that the "iter" slot in the
  type is "iternext", and should use the identity getiter.
- MP_TYPE_FLAG_ITER_IS_CUSTOM: This means that the "iter" slot is a pointer
  to a mp_getiter_iternext_custom_t instance, which then defines both
  getiter and iternext.

And a third flag that is the OR of both, MP_TYPE_FLAG_ITER_IS_STREAM: This
means that the type should use the identity getiter, and
mp_stream_unbuffered_iter as iternext.

Finally, MP_TYPE_FLAG_ITER_IS_GETITER is defined as a no-op flag to give
the default case where "iter" is "getiter".

Signed-off-by: Jim Mussared <jim.mussared@gmail.com>

extmod/vfs_lfsx_file.c

@@ -223,11 +223,9 @@ STATIC const mp_stream_p_t MP_VFS_LFSx(fileio_stream_p) = {
 MP_DEFINE_CONST_OBJ_TYPE(
     MP_TYPE_VFS_LFSx_(_fileio),
     MP_QSTR_FileIO,
-    MP_TYPE_FLAG_NONE,
+    MP_TYPE_FLAG_ITER_IS_STREAM,
     MP_TYPE_NULL_MAKE_NEW,
     print, MP_VFS_LFSx(file_print),
-    getiter, mp_identity_getiter,
-    iternext, mp_stream_unbuffered_iter,
     protocol, &MP_VFS_LFSx(fileio_stream_p),
     locals_dict, &MP_VFS_LFSx(file_locals_dict)
     );
@@ -242,11 +240,9 @@ STATIC const mp_stream_p_t MP_VFS_LFSx(textio_stream_p) = {
 MP_DEFINE_CONST_OBJ_TYPE(
     MP_TYPE_VFS_LFSx_(_textio),
     MP_QSTR_TextIOWrapper,
-    MP_TYPE_FLAG_NONE,
+    MP_TYPE_FLAG_ITER_IS_STREAM,
     MP_TYPE_NULL_MAKE_NEW,
     print, MP_VFS_LFSx(file_print),
-    getiter, mp_identity_getiter,
-    iternext, mp_stream_unbuffered_iter,
     protocol, &MP_VFS_LFSx(textio_stream_p),
     locals_dict, &MP_VFS_LFSx(file_locals_dict)
     );