Improve debug str generator

Generates more optimized code per https://rust-lang.github.io/rust-clippy/master/index.html#format_collect

Author: nyurik

bindgen-tests/tests/expectations/tests/derive-bitfield-method-same-name.rs

@@ -194,8 +194,13 @@ impl<'a> ImplDebug<'a> for Item {
                             self.#name_ident
                                 .iter()
                                 .enumerate()
-                                .map(|(i, v)| format!("{}{:?}", if i > 0 { ", " } else { "" }, v))
-                                .collect::<String>()
+                                .fold(String::new(), |mut output, (i, v)| {
+                                    if i > 0 {
+                                        output.push_str(", ");
+                                    }
+                                    let _ = write!(output, "{:?}", v);
+                                    output
+                                })
                         }],
                     ))
                 }