Merge #2551

2551: Refactor tt::Subtree delimiter r=matklad a=edwin0cheng

Refactor to :
```
#[derive(Debug, Clone, PartialEq, Eq, Hash, Default)]
pub struct Subtree {
    pub delimiter: Option<Delimiter>,
    pub token_trees: Vec<TokenTree>,
}

#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
pub enum Delimiter {
    Parenthesis,
    Brace,
    Bracket,
}
```

Co-authored-by: Edwin Cheng <edwin0cheng@gmail.com>