[MLton] Bug in monomorphise(?)

[Stephen Weeks]

> I've checked in a fix to the monomorphise pass.  AFAICT, this bug has been
> latent since the original mlton-19990319 release!

Indeed.  This easily wins for bugfix of the longest standing bug.  I
checked through my old snapshots (long before MLton was under revision
control) and see that the bug was introduced between November 13, 1998
and December 17, 1998.

Speaking of history, MLton is almost 10 years old!  The earliest
snapshot I have is dated October 15, 1997, and coding had started
around August 1, 1997.