time bug

[Matthew Fluet]

> I got that time bug again.  I've given up on stracing self-compiles
> (the compiler get < 50% of the CPU time and it's just not worth it).
> Anyways, here's the Failure message:
>
> mlton: traceBatch: t' - t::-: t1 = 597.600 t2 = 598.590

In trying to track down the time bug, I've been running repeated
self-compiles overnight, bouncing back and forth between stracing and not,
and saving any logs with errors.  I got another one without stracing:

mlton: Time:traceBatch: t' - t::-: t1 = 590.720 t2 = 591.710

Something that might be interesting, both times t1 and t2 differ by 0.99.

Also, I discovered a minor bug in the code I sent out a few days ago.  The
handle Time wasn't handling the Time exception, because that exception is
not at the top level.  In fact, it's not even exported by
/src/lib/mlton/basic/time.sig.  I added the exception to the library TIME
sig and the exception above really is the Time exception.