Fix some dialyzer issues

[lukaszsamson]

Besides those two, dialyzer is also complaining that those will never match

  defp eat_at(parser, token, 0) when is_atom(token) do
    eat(%{token => true}, parser)
  end

  defp eat_at(parser, token, idx) when is_atom(token) do
    eat_at(parser, %{token => true}, idx)
  end

[mhanberg]

Superceded by #61

I cherry picked your commits that are still used