[m-dev.] cleaning up the item list

[Zoltan Somogyi]

In september, we discussed how the code dealing with the item list
is a big mess. After my changes to the implementation of mutables,
I realized that those changes eliminated one of the bigger obstacles
in the way of such cleanups: the way we threaded updates to the
current status through pretty much all of the many modules of the
make_hlds package. So I started work on making the structure
of the item list, which is currently undocumented and not reflected
in the types, apparent in the types themselves. It took a lot of work,
but I am now about half way through this cleanup, and I think it
quite likely that I will be successful this time. But to make this
possible, I am asking everyone else not to commit any changes
that affect code working on the item list, or items themselves,
or statuses, until I am done.

Zoltan.