chore: manual inline formatting

Minor manual cleanup - inlined many format args.  This makes the code a bit more readable, and helps spot a few inefficiencies and possible bugs.  Note that `&foo` in a `format!` parameter results in a 6% extra performance cost, and does not get inlined by the compiler (yet).

tests/by-util/test_rm.rs

@@ -333,8 +333,7 @@ fn test_rm_verbose_slash() {
     at.touch(file_a);
 
     let file_a_normalized = &format!(
-        "{}{}test_rm_verbose_slash_file_a",
-        dir,
+        "{dir}{}test_rm_verbose_slash_file_a",
         std::path::MAIN_SEPARATOR
     );