[MLton] unused variables in MLton

[Henry Cejtin]

One  interesting  point  Rico  brought  up when I was showing him the current
experimental version: on my ancient machine (a 400 MHz CPU), MLton grinds for
9.70  CPU  seconds  before  it  even opens the files provided in command line
arguments.  This must be reading in the basis library and the like.   In  the
old  days  this  wasn't too bad of a problem because one did not use MLton to
develop code, but now it is a bit of a pain.  It seems to me that this  is  a
strong  argument  for putting back the save-world after the basis library has
been read in.