Avoid naming variables str

This renames variables named `str` to other names, to make sure `str`
always refers to a type.

It's confusing to read code where `str` (or another standard type name)
is used as an identifier. It also produces misleading syntax
highlighting.

library/std/src/process.rs

@@ -1283,13 +1283,13 @@ impl fmt::Debug for Output {
     fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
         let stdout_utf8 = str::from_utf8(&self.stdout);
         let stdout_debug: &dyn fmt::Debug = match stdout_utf8 {
-            Ok(ref str) => str,
+            Ok(ref s) => s,
             Err(_) => &self.stdout,
         };
 
         let stderr_utf8 = str::from_utf8(&self.stderr);
         let stderr_debug: &dyn fmt::Debug = match stderr_utf8 {
-            Ok(ref str) => str,
+            Ok(ref s) => s,
             Err(_) => &self.stderr,
         };