[MLton] adding -ann switch

[Matthew Fluet]

> I'm wondering if we could replace the following command-line switches
>
>     -allow-export {false|true}
>     -allow-import {false|true}
>     -dead-code {true|false}
>     -sequence-unit {false|true}
>     -warn-match {true|false}
>     -warn-unused {false|true}
>
> with a single switch
>
> 	-ann <ann>

I would call it something other than just -ann.  Maybe -default-ann.
My concern is that the behavior with respect to -{enable,disable}-ann
might be a little confusing.