item = GetItem(list, idx)
idx in the list specified by list. If idx is -1,
the last list item will be returned. If idx is out of range, Nil
will be returned.
Note that you normally shouldn't use this function because you can access list
elements quicker by just using the []-operator. The only case where using this
function has a real speed advantage is when you want to get the last item of an
optimized list and you don't know the number of items in the list. In that case,
using GetItem() is faster than calling ListItems() first.