support for `eval` command + examples

currently I cant figure out the proper way to call `eval`:

```elixir
test "should eval expression", %{t: t} do
    Tarantool.Api.eval(t, %{expr: 'return "hola"', tuple: [""]}) |> IO.inspect()  
end
>> {:error, 32788, "Invalid MsgPack - packet body"}
```

Any help / docs on this? 