[MLton] MinGW port

[Stephen Weeks]

Thanks for the thoughts Mike. 

> My opinion, based on observation of a number of large cross platform
> projects, is that attempts to emulate Unixisms (eg Posix) which
> don't fit naturally on Windows is a bad idea which leads ultimately
> to a lot of wasted time and effort.  If you want Posix on Windows,
> use Cygwin and remember that the Posix bugs/differences in behaviour
> under Cygwin are still there despite many person years of hard work
> and thought by some clever people.

I agree.  I think it's OK to enable simple functionality (like
Posix.FileSys.rmdir) that has an easy mapping to windows.  I'll be
more wary as the mapping gets more complex.