[MLton] array flattening

[Matthew Fluet]

> I  am  very  interested  in what the array flattening thing will do since its
> lack is one of the biggest space inefficiencies that I see in generated MLton
> code.   Despite  that,  I  guess that it will cause some things to get worse.
> E.g., if I create an array with
>         Array.array (1000000, (0, 0))
> then it currently that takes only 4 bytes per element, while, I assume,  your
> proposed optimization will flatten out things, so it will be 8 bytes per.

Worse in what way?  Really, the former is 4 + (4 + 2 * 4) bytes per
element while the later will be 8 bytes per.  So, you are definitely
winning overall.