feat: support recursive nesting

[faervan]

close #50

    #[test]
    fn recursive_nesting() {
        #[derive(TomlExample, Default, Debug, Deserialize, PartialEq)]
        struct Outer {
            #[toml_example(nesting)]
            _middle: Middle,
        }
        #[derive(TomlExample, Default, Debug, Deserialize, PartialEq)]
        struct Middle {
            #[toml_example(nesting)]
            _inner: Inner,
        }
        #[derive(TomlExample, Default, Debug, Deserialize, PartialEq)]
        struct Inner {
            _value: usize,
        }
        let example = Outer::toml_example();
        assert_eq!(toml::from_str::<Outer>(&example).unwrap(), Outer::default());
        assert_eq!(
            example,
            r#"[_middle]
[_middle._inner]
_value = 0

"#
        );
    }

This causes the toml_example_with_prefix method to change, tho this method should usually only be used via the TomlExample derive macro anyway.

To-do

• add additional test(s)

I don't think updating the documentation is necessary, as one would expect this to work already

[faervan]

I also noticed I made a mistake when I implemented support for flatten, it would put the flattened items at the end with the other nested items, even tho flattened items should be treated as non-nested due to the nature of flattening.
Fixed it and added a test here as well.

[yanganto]

Thanks, I think it is ready to go.

The code is clean straightforward enough, and this is just a picky nit suggestion.

If you don't mind, may we add some doc string on fn toml_example_with_prefix of traits. Although this function is not really for public usage and called by macro, the document still good and may helpful for developer to keep maintaining.