It will not detect 0-ary functions defined without parens

This test suite will fail.

```elixir
  test "empty args" do
    code = """
    defmodule Foo do
      def bar do
        2137
      end
    end
    """

    assert {:ok, 2137} = MiniElixir.eval(code, Foo, :bar, [])
  end
```