profiling

[Stephen Weeks]

> Turns out that my "appropriate" Plus and Minus functions were too naive.
> In particular, I wasn't keeping the usec field of the ru_utime and
> ru_stime fields in the range 0 <= usec < 10^6.  This was really only a
> problem with the gc, which could end up with a negative usec, which raises
> Time in Time.fromMicroseconds.  (I guess the usec field could also
> overflow, although that would just make the time recorded inaccurate.)
> 
> Updated gc.c is at:
> http://www.cs.cornell.edu/People/fluet/MLton/gc.tgz

Works great for me.  I just built G1 and G2 without event.  I also made a couple
of cosmetic changes to gc.c and also made the change to
src/basis-library/posix/proc-env to make Posix.ProcEnv.times use word instead of 
int.