[MLton] admits equality

[Matthew Fluet]

Would it be more consistent if the type error message for failing to admit 
equality used the "[...]" convention of other type error messages.  That 
is, instead of

Error: z.sml 1.16.
   Type t admits equality in signature but not in structure.
     not equality: real * _ * _
compilation aborted: parseAndElaborate reported errors

have

Error: z.sml 1.16.
   Type t admits equality in signature but not in structure.
     not equality: [real] * _ * _
compilation aborted: parseAndElaborate reported errors