Rewriting the Rewrite trait

Rustfmt, the de facto standard tool for formatting Rust code, has been reported that it occasionally fails to format silently. One of the major causes is that the internal formatting mechanism loses the context when issues occur during formatting. While rustfmt heavily relies on the Rewrite trait to format various AST structures, the rewrite method, the only method that consists of the Rewrite trait, returns Option. This makes it hard to track how and why formatting failed. To improve handling of silent failures and allow rustfmt to retry formatting under more contexts, this project aims to refactor the Rewrite trait so that errors occurring in formatting can be propagated.