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/sys_common/wtf8.rs

@@ -204,8 +204,8 @@ impl Wtf8Buf {
     ///
     /// Since WTF-8 is a superset of UTF-8, this always succeeds.
     #[inline]
-    pub fn from_str(str: &str) -> Wtf8Buf {
-        Wtf8Buf { bytes: <[_]>::to_vec(str.as_bytes()), is_known_utf8: true }
+    pub fn from_str(s: &str) -> Wtf8Buf {
+        Wtf8Buf { bytes: <[_]>::to_vec(s.as_bytes()), is_known_utf8: true }
     }
 
     pub fn clear(&mut self) {