rust: str: normalize imports in str.rs

Clean up imports in `str.rs`. This makes future code manipulation more
manageable.

Signed-off-by: Andreas Hindborg <[email protected]>
Reviewed-by: Alice Ryhl <[email protected]>

Author: Andreas Hindborg

rust/kernel/str.rs

@@ -3,10 +3,11 @@
 //! String representations.
 
 use crate::alloc::{flags::*, AllocError, KVec};
-use core::fmt::{self, Write};
-use core::ops::{self, Deref, DerefMut, Index};
-
 use crate::prelude::*;
+use core::{
+    fmt::{self, Write},
+    ops::{self, Deref, DerefMut, Index},
+};
 
 /// Byte string without UTF-8 validity guarantee.
 #[repr(transparent)]