change to MLton.World.saveThread

[Henry Cejtin]

I  have  been  once again looking at what needs to be done to make saveThread
useful for  staged  computation.   I.e.,  I  want  saveThread  to  create  an
executable which includes both the original executable and the heap.  I think
I now know how to do this.  I'll see if I can get it to work in the next  few
days.

My  notion  would  be  that  there  would be NO option to allow the resulting
executable to go back to the `original' world.  It can ONLY restart the heap.
The  point  is  that  one  might  do  fancy multi-staged worlds, and then the
question would be why single  out  the  `original'  world.   It  is  just  as
legitimate to want to go part way back.

One  disadvantage  of  this  (I think it is minor) is that the original a.out
file is replicated in the saved world.  I view this as a very minor loss.  It
is  only a concern if you want to go back (and, I suppose, for the transitory
requirement of disk space).

Does this seem useful to people?  Is the change OK?