[MLton] Cygwin->Mingw32: patch + future

[Stephen Weeks]

> > > What sort of problems does this cause? [cygwin+mmap]
> 
> I still would like to know the answer to this.
> It seemed to work as reliably as linux for me.
...
> Does it just run out of memory and die, or what?

A call to mmap fails, and MLton's runtime reports an out-of-memory
error.

> I would think fork() is more important than the ability to address huge
> amounts of memory. Which do more programs use?

Both are useful.  The best solution I can think of is to add a switch
that governs whether mmap or VirtualAlloc is used.