[MLton] cvs commit: improved exception history for Overflow

[Matthew Fluet]

> I ended up going for a two-piece solution that didn't require changing
> any ILs.  The elaborator wraps an Enter/Leave around each raise
> expression, just as it does around each function body.  The (still
> horribly misnamed) defunctorizer wraps an Enter/Leave around each
> raise {Bind,Match} that the match compiler generates.  Fortunately, we
> already had the source position info there for the warnings generated
> by match compilation.

These extra Enter/Leave's should probably be predicated upon
Control.exnHistory, else they will add a lot of source positions to
-profile count.

Alternatively, if one thinks that counting raises would be of interest, we 
could add  -profile-raise {false|true}  which is implicitly enabled by 
-const 'Exn.keepHistory true' and then the elaborator/defunctorizer would 
predicate upon Control.profileRaise.