Add `Node` union

Add a new Node union that is an enum over all node types. This can be useful when implementing methods that can operate on any node. For example, Ruff's formatter has (roughly) the API format(node: AnyNode) -> String

I haven't figured out the full requirements yet, and I don't know yet if we'll need both the owned and reference versions:

#[derive(Debug, Clone, PartialEq, Eq, Hash)]
enum Node {
	IfStmt(ast::IfStmt),
	ConstantExpr(ast::ConstantExpr),
	...
}

#[derive(Debug, Copy, Clone, PartialEq, Eq, Hash, is_macro)]
enum NodeRef<'a> {
	IfStmt(&'a ast::IfStmt),
	ConstantExpr(&'a ast::ConstantExpr)
	...
}

Note: An alternative naming more in line with Path and PathBuf would be to name the types NodeBuf (owned) and Node (reference)

The enums should have the following basic methods:

• if_stmt(self) -> Option<ast::IfStmt>
• as_if_stmt(&self) -> Option<&ast::IfStmt>
• const is_if_stmt(&self) -> bool

Node could also implement AsRef that returns a NodeRef

I may have time to work on this sometime soon but I wanted to put this up for discussion first to get feedback.

检查现有的 AST 节点类型，并比较提议的拥有型 Node 和引用型 NodeRef 设计。在实现枚举、转换方法和类型检查之前，解决命名和所有权要求。支持的节点类型和所需的 API 达成一致并由测试覆盖后，即视为完成。