Add runSTArray for efficiently constructing arrays using ST.

[Dretch]

Haskell has this already. Also add a test for withArray.

I think this makes sense - please reject this pull request if not!

[hdgarrood]

As it happens, we previously had this (and not unsafeFreeze), but then we removed it at the same time we added unsafeFreeze; of course you can reconstruct this function in terms of unsafeFreeze (as you have here). I didn't realise at the time that this function is useful to have in addition to unsafeFreeze though, since of course this function allows you to obtain the benefits of unsafeFreeze while retaining more type safety. See #87, #89

So 👍 from me, although I would suggest calling it just run, as we've tried to avoid module name / function name duplication in this module.

[Dretch]

@hdgarrood it is interesting to know the history behind this, thanks.

I have renamed runSTArray to run, as requested.

[hdgarrood]

Thanks!