Style tweaks to the newgame_undo patch.

Style tweaks to the newgame_undo patch.

I've renamed the new midend variables to match my usual naming
convention of using 'size' for the total buffer size and 'len' for the
amount of currently used space (and a couple of other variables to
match those in turn), partly for consistency and also because the name
'midend_undo_used' made me half-expect it to be a boolean. The buffer
itself is called 'midend_undo_buf', again to avoid making it sound
like a function or flag.

Buffer growth is still geometric but less aggressive (factor of 5/4
each time instead of 2), in the hope of wasting less memory on low-RAM
platforms; newgame_undo_deserialise_read should have been static, and
now is; newgame_undo_buf is initialised using NULL rather than 0 so it
doesn't look like an integer, and is freed with the rest of the
midend.

And I think we _should_ enforce by assertion that midend_deserialise
didn't return an error, because there's no reason it ever should in
this situation (unlike when reading from a file, where the user might
have provided the wrong file or a corrupted one). This immediately
allowed me to spot a bug in the existing deserialisation code :-)