GC tweaks

[Henry Cejtin]

As  a  hack  experiment,  I  combined gc.c, GC_size.c and GC_world.c into one
file, then replaced all the function  pointer  arguments  with  integers  and
replaced  the  calls to switch statements.  I checked that if I inlined these
functions then gcc would figure it all out and inline everything.  There  was
some  uglyness  with  getting  the order right in the source, but it seems to
have worked.  I think used this gc-mongo.o to make a new  mlton-compile  (and
world.mlton) and then did a self compile with it.  On my machine, Red Hat 7.1
with 512 meg of RAM, the time are:

            total           GC time
    old     879.42          279.29
    new     835.31          243.90

The `old' one includes my previous GC hacks.  Thus we get 5% over all and 15%
of  GC  time.  Yawn.  The code size increase was under .1%.  My guess is that
there really is a lot more to get here, but clearly it will take a  real  re-
write  of the GC, and as Stephen said, in that case making it generational is
probably the way to go.