Refactor to make me->newgame_undo a small struct.

Refactor to make me->newgame_undo a small struct.

The three related buf/len/size fields are now a sub-structure of the
main midend, and the serialise/deserialise functions that address
those fields now take a pointer to that sub-structure rather than to
the midend as a whole. This will make it easy for me to drop in a
second substructure alongside it, for redo, and reuse those read and
write functions by just changing the context pointer.