final GC tweak for this version

[Stephen Weeks]

> Here  is  a  hacked up version of gc.c.  The changes consist of moving things
> around and making more things inline.  The result isn't very  much:  for  the
> self  compile it sped up the whole thing byt 2.7%, speeding up the GC time by
> 9.1%.  I would probably be just as happy not including it, but I leave it  up
> to you.

I've put it in.  It does add 3K to every executable, but oh well.  Here's the
time difference I see for a self compile.

old: MLton finished in 363.24 + 163.56 (31% GC)
new: MLton finished in 365.52 + 149.44 (29% GC)

Not bad.  Anyways, it passed all the regressions and I am now doing the final
benchmark run.