Add ok binding to all functions.

NOTE this is an experimental feature, it will be removed or added permanently for 2.0 release

Example

defmodule MyApp do
  use OK.Kernel

  def checkout(user_id, cart_id) do
    user <- fetch_user(user_id)        # `<-` will bind user when fetch_user returns {:ok, user}
    cart <- fetch_cart(cart_id)        # `<-` will shortcut to else clause if returned {:error, reason}
    order = checkout(cart, user) # `=` allows pattern matching on non-tagged funcs

    order.invoice_id
  else
    :user_not_found ->
      IO.puts("No user for user_id: #{user_id}")
      nil
    :user_not_found ->
      IO.puts("User has no cart")
      nil
  end

end

TODO move examples to OK.def/2