New language feature: array-types

[Don Stewart]

lennart:
> As for array updating, there are many ways to improve the O(n) update.
> You can use a tree representation and get O(log n) for all operations.
> You can use the array single threaded in the ST monad and get all the
> usual array operation complexities.

Or use a history/transaction list to average out the copy cost, or use
fusion to minimise the updates required. 

Making pure arrays efficient is a lot of fun, but it's a library issue,
not a language one, necessarily.

-- Don