new backend integration

[Matthew Fluet]

Steve, glad to hear that the integration was successful.  There's plenty
to think about and work on outside of the primApp function, plus I have a
lit of the names in the other e-mail, so I probably don't need a snapshot
until Monday.  However, if you've got a copy of mlton.c from the
self-compile around, could you post that on the web?  I'm trying to track
down how some of the different constructs are being used, and a large
program would be useful to look at.