[MLton] array flattening

[Henry Cejtin]

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.