mlton usage messages

[Stephen Weeks]

> I vote for having the basis library take into account the -detect-overflow
> flag.  I can't imagine only wanting to turn it off in the +, - and * and
> wanting it left on for div and quot.

Here are all the functions in the basis library that can raise overflow.

Int.{div, quot, +, -, *}
IntInf.toInt
Real.{checkFloat, toInt}
Word.{fromString, scan, toInt}

I guess it makes sense to turn it off for all Int functions but leave it on for
others?