Simplify Enum.Iterator, second round

I feel this is very important, so I made what @yrashk proposed, it works although the documentation there would need some love, but this is just a draft to have an actual proposal with code backing it up, so the same goes for missing tests.

So yeah, I want this, anything against it that isn't code _complexity_ (it's less than 100 lines which would make implementing iterators way easier)?

Here's a stupid example:

```elixir
defrecord LOL, [] do

end

defimpl Enum.Iterator, for: LOL do
  use Enum.Iterator.Defaults

  def iterator(_) do
    { fn _ -> :stop end, { 1, nil } }
  end
end

IO.inspect Enum.count(LOL[])      # => 1
IO.inspect Enum.member?(LOL[], 1) # => true
IO.inspect Enum.member?(LOL[], 2) # => false
IO.inspect Enum.empty?(LOL[])     # => false
```