[MLton] mingwows

[Stephen Weeks]

> I support the idea of an mlton executable - I have never understood
> why launching a compiler should require two steps instead of one.

The reason we do it is to have a file that can be easily edited by
hand and contains all of the platform-specific paths and options.
This allows people to easily adapt a MLton installation to their
situation without having to recompile MLton (which is, relative to
editing a script, quite painful).

> If you choose to use a driver, it might as well be a compiled SML
> program rather than a platform specific script.

This does seem like a reasonable way to go, as compiling a small
MLton/SML program is not that much more painful than editing a script.
We'll consider as a way forward after this release.