Access protocol for all lists of tuples

This is incredibly useful for dealing with things like JSON.

For example, in my app I assumed this would work:

```elixir
rpc = JsonRpc[jsonrpc: json["jsonrpc"],
              id: json["id"],
              method: json["method"],
              params: json["params"]]
```

Other possibilities:

```elixir
star_ratings = [{1.0, "★"}, {1.5, "★☆"}, {2.0, "★★"}]
star_ratings[1.5] #=> "★☆"
```