[MLton] MLton.Vector.create

[Henry Cejtin]

As  I  said  when  we  spoke,  I definitely like it.  I would make it so that
calling sub after create returns also raises Fail.

Also, I wonder if it is worth while having something just  like  create  that
takes  a  tabulate function instead of a single element for the initial value
of the array/vector.  I'm thinking of cases where there is no  natural  value
for  all the elements, but only a different one for each element.  An example
might be when each entry needs its own ref cell some where.

Of course one can always fake this from the given create, but  it  is  pretty
ugly.   (In fact, I guess that the case where n = 0 actually might be hard to
even fake if the tabulate function diverges.)