[MLton] free type variables in datatype decs

[Andreas Rossberg]

Claudio,

man, such a vicious example, half an our after I made a release. You did
that on purpose... :-)

Of course, Hamlet incorrectly rejects

  fn r => let datatype t = C in r := C end

And frankly, I have no clue how one could possibly implement this detail of
scoping accurately without resorting to a second pass. I very much feel like
considering this a practical bug on side of the Definition...

  - Andreas