profiling

[Matthew Fluet]

> Hopefully the following code clarifies what I had in mind.

I implemented a version based on Steve's code.  I'm not entirely happy
with it; in particular, it would probably be desirable that when not
compiling with -p, there are no vestiges of profiling in the code.  But,
the various isFreed ref cells can't be compiled away.

I posted a patch against the current sources at
http://www.cs.cornell.edu/People/fluet/MLton/mlton.src.tgz
There were only changes to src/runtime and src/basis-library, so checking
it out doesn't need a whole recompile.  Just re-make the runtime and rerun
mlton-compile against the new basis-library.