flatten_map/1 does not flatten lists

Given the following structure:
```elixir
%{items: [%{id: 1}, %{id: 2}], another: %{key: :value}}
```

I would expect the following result:
```elixir
%{another__key: :value, items__0__id: 1, items__1__id: 2}
```

But we actually get:
```elixir
%{another__key: :value, items: [%{id: 1}, %{id: 2}]}
```

I wonder if we should have a `flatten_list/1` and `flatten_data/1` which would combine flatten_list + flatten_map.