Add a createT analogue for arrays

[David Feuer]

The vector package offers

createT :: Traversable t
  => (forall s. ST s (t (MVector s a)))
  -> t (Vector a)

This is a generalization of create, which is very similar to runSTArray in
the array package. I suggest we add functions

runSTArrays :: Traversable t
  => (forall s. ST s (t (STArray s i e)))
  -> t (Array i e)

runSTUArrays :: Traversable t
  => (forall s. ST s (t (STUArray s i e)))
  -> t (UArray i e)

Why do I think it's worth the trouble? While it's occasionally useful to
create multiple arrays of the same type in one go, I think the Maybe and
(a,) Traversable instances, and their compositions, are likely more
important. I can use (a,) to record some extra information while building
an array. I can use Maybe to give up and not produce an array. And the
compositions let me do both in different ways.
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