[MLton] free type variables in datatype decs

[Andreas Rossberg]

> MLton has a pass, scope inference, that runs before type inference and
> determines the scope of each implicitly-scoped type variable.  That
> seems to be what 4.6 specifies, except that 4.6 doesn't state as
> clearly as it might that every type variable must be scoped somewhere.

In fact, that is not the case: exception declarations may contain free type
variables, as long as they do not propagate to the toplevel. For example,
the program

  structure S : sig end = struct exception E of 'a end

is legal SML, but rejected by MLton.

Of course, this is of no practical use whatsoever, another artefact of the
way the Definition handles tyvar scoping.