rust: str: use the "kernel vertical" imports style

Convert the imports to use the "kernel vertical" imports style [1].

No functional changes intended.

Link: https://docs.kernel.org/rust/coding-guidelines.html#imports [1]
Reviewed-by: Gary Guo <gary@garyguo.net>
Link: https://patch.msgid.link/20260609104152.261145-1-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/kernel/str.rs

@@ -3,14 +3,28 @@
 //! String representations.
 
 use crate::{
-    alloc::{flags::*, AllocError, KVec},
-    error::{to_result, Result},
-    fmt::{self, Write},
-    prelude::*,
+    alloc::{
+        flags::*,
+        AllocError,
+        KVec, //
+    },
+    error::{
+        to_result,
+        Result, //
+    },
+    fmt::{
+        self,
+        Write, //
+    },
+    prelude::*, //
 };
 use core::{
     marker::PhantomData,
-    ops::{Deref, DerefMut, Index},
+    ops::{
+        Deref,
+        DerefMut,
+        Index, //
+    }, //
 };
 
 pub use crate::prelude::CStr;