In compact mode, we currently can not support zero-allocation moves
since some pointer assignments required during the move need to allocate
hash table slots.

This is mostly applicable to xml_document_struct::first_child, since the
pointer to this element is used as a hash table key, but there are some
contrived cases where parents of root's children need a hash slot and
didn't have it before.

These cases can be fixed by changing the compact encoding to be a bit
more move friendly, but for now it's easier to handle the error and
throw/return during move.

When this happens, the source document doesn't change.