[MLton] Representation of strings in the FFI

[Stephen Weeks]

> I take it, I should use GC_arrayNumElementsp at the C level.
> 
> /* The array size is stored before the header */
> static inline uint* GC_arrayNumElementsp (pointer a) {
...
> static inline int GC_arrayNumElements (pointer a) {

Yes, you can use either of these.  Although you could use the same
trick I just mentioned too, with the drawback that only monomorphic
values may be exported

val _ = _export "intArrayLength": int array -> int;
        Array.length