[MLton] Bug in parseAndElaborate: unhandled exception: Subscript

[Stephen Weeks]

> The latest MLton built from SVN sources has a bug in the parseAndElaborate
> pass.
...
> signature X =
>    sig
>       type x = unit
>    end
> 
> structure X :> X =
>    struct
>       type 'a x = unit
>    end
...
> unhandled exception: Subscript

Thanks.  Bug fixed.  MLton now reports (as it used to)

  Error: z.sml 6.16.
    Type x has arity 1 in structure but arity 0 in signature.

This problem points to our severe need to integrate error cases into
our regressions.  I think the front end has stabilized to the point
where it wouldn't be too much noise to find out every time that error
messages change.  Any takers?  There's already a fair number of tests
in regression/fail.  The main thing is to do any necessary cleanup,
add the ".ok" files, and add some lines to the bin/regression script.