self compile time

[Stephen Weeks]

After fixing a performance bug in the match compiler and a nasty space leak that 
was basically slowing everything down (due to excess GC), there has been a
substantial improvement in self compile time.  We're almost under 8 minutes!
Here is the breakdown for the latest run.

pre codegen	184
codegen		285
compile C	  6
assemble	 13
link		  1
                ---
total		489

GC	        174

It would be nice to use this version of MLton for the numbers in the paper,
since we will be reporting compile times for large benchmarks (I hope).  I also
have a couple of new benchmarks we should try: zebra (300 lines) and hamlet (23k 
lines).  It might be good to try the kit benchmark again as well.  That would
give us four benchmarks (yacc, hamlet, kit, mlton) with > 5000 lines.