Refactoring here-doc content filling mechanism

[magicant]

The current mechanism requires conversion of the entire AST data only to insert here-doc contents into the AST. Can we avoid this behavior by using RefCell and only modifying the relevant part of the AST?

---

The new type definitions will be:

pub struct HereDoc {
    pub delimiter: Word,
    pub remove_tabs: bool,
    pub content: RefCell<Text>,
}
pub enum RedirBody {
    Normal {
        operator: RedirOp,
        operand: Word,
    },
    HereDoc(Rc<HereDoc>),
}

---

Todo:

• Wrap HereDoc::content in RefCell
• Wrap RedirBody::HereDoc's value in Rc
• Change Parser::unread_here_docs to a list of Rc<HereDoc>
• Remove Parser::read_here_docs
• Remove Fill
• Remove unnecessary type parameters from all AST types