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.

既存のASTノード型を確認し、提案されている所有型のNodeと参照型のNodeRefの設計を比較してください。enum、変換メソッド、型チェックを実装する前に、命名要件と所有権要件を解決してください。サポート対象のノード型と必要なAPIについて合意され、テストで網羅されていれば完了です。