[MLton] front-end unhandled exception

[Stephen Weeks]

> [fluet@cfs39 bug 23]% cat z.sml
> signature T =
>   sig
>      val x : 'a t
>   end
> [fluet@cfs39 bug 24]% mlton z.sml
> Error: z.sml 3.17:
>    Undefined type t.
> Vector.map2

Fixed.