make KeyIterator and ValueIterator more array-like

When have d = Dict(:foo => 12.34, "bar" => 567) and you write keys(d) you get a Base.KeyIterator object, which is fast and cheap to construct from the dictionary d. You can write collect(keys(d)) to get an array of keys, but you have to do this unfortunately often. It would be a good self-contained project to add more array-like behaviors to KeyIterator and the corresponding ValueIterator types so that things like keys(d)[1] and values(d)[end] work as expected.