last for OrderedDict/KeyIterator/ValueIterator

[oxinabox]

first works on all of these, but last doesn't

julia> dd =  OrderedDict(1=>2, 3=>4)
OrderedDict{Int64, Int64} with 2 entries:
  1 => 2
  3 => 4

julia> first(dd)
1 => 2

julia> last(dd)
ERROR: MethodError: no method matching lastindex(::OrderedDict{Int64, Int64})
...

But you can get them via:

julia> last(dd.keys)
3

julia> last(dd.vals)
4

But that is going in to the internals.

This should be a nice easy PR.
Just need to dispatch on Base.ValueIterator{OrderedDict{<:Any, <:Any}} (similar for the KeysIterator) and look at the backing dict, and then look at it's dict.vals (similarly dict.keys)

[pri1311]

Surprising! I have previously added a PR for the same - #94 also works locally for me