[MLton] Monadic MLton.Vector.create with update

[Henry Cejtin]

Before I would go for VM remapping tricks for large arrays, I would think that
a bigger win would be big-bag-of-pages tricks for small objects.  I.e., if
you have lots of objects in the heap that are of the same type (in the sense
of header words) and small, then put them in their own region and get rid of
the header words, using an address range check instead.

It can save a very large amount of space, but complicates the mutator runtime
since you still would have to handle objects of that type in the nursery.