[MLton] Two questions about FFI types

[Tom 7]

Thanks for your quick answers, both of you!
I updated the FFI types page on the wiki to reflect the read-onlyness of 
strings and vectors.

> This is not safe.  MLton's optimizer and runtime know that vectors and
> ...
>  _prim "Array_toVector": 'a array -> 'a vector;

I was sorta hoping that this would be the answer.

  - Tom