clippy fixes for librustdoc

fixes clippy warnings of type:
match_like_matches_macro
or_fun_call
op_ref
needless_return
let_and_return
single_char_add_str
useless_format
unnecessary_sort_by
match_ref_pats
redundant_field_names

src/librustdoc/html/toc.rs

@@ -132,7 +132,7 @@ impl TocBuilder {
                 }
                 Some(entry) => {
                     sec_number = entry.sec_number.clone();
-                    sec_number.push_str(".");
+                    sec_number.push('.');
                     (entry.level, &entry.children)
                 }
             };