Unseparated punctuations should have `Spacing::Joint`

The document of [proc_macro::Spacing](https://doc.rust-lang.org/stable/proc_macro/enum.Spacing.html#variant.Joint) says:

> * When a Punct is immediately followed by another Punct without a whitespace. E.g. `+` is Joint in `+=` and `++`.

However, `quote!(++)` generates two sepalated `+` tokens which have `Spacing::Alone` and `quote!(++).to_string()` is `"+ +"`.