[MLton] IntInf_to_WordVector semantics

[Daniel Harvey]

We've implemented a variety of the primitive operations. But we are unsure
about IntInf_to_WordVector. Looking at the signature of WordXVector it
looks like the only way to create one is to use the fromString function.
We are wondering how exactly any given number should be translated to a
string to produce the correct word vector. Assuming that is the right
thing to do at all. It would seem logical that we would just produce the
number as a string, but the, IntInf -> string -> word vector, sequence
seems odd.