rust: use while let in trim_whitespace implementation

originally proposed in a comment at #911 (comment)

- using while let allows us to combine all tests and
  the reminder construction  into one pattern
- without slice indexing and as such  the use of the `Index` trait
  we can now also make the function `const`
- a quick comparison on [Compiler Explorer](https://godbolt.org/z/Ex5dT8ffr)
  shows no changes in the resulting instructions,
  at least for the checked constellation

Signed-off-by: Bennet Bleßmann <[email protected]>

rust/kernel/str.rs

@@ -608,16 +608,12 @@ macro_rules! fmt {
 /// assert_eq!(trim_whitespace(b"    foo"), b"foo");
 /// assert_eq!(trim_whitespace(b"  foo  "), b"foo");
 /// ```
-pub fn trim_whitespace(mut data: &[u8]) -> &[u8] {
-    while !data.is_empty() && (data[0] == b' ' || data[0] == b'\t' || data[0] == b'\n') {
-        data = &data[1..];
-    }
-    while !data.is_empty()
-        && (data[data.len() - 1] == b' '
-            || data[data.len() - 1] == b'\t'
-            || data[data.len() - 1] == b'\n')
-    {
-        data = &data[..data.len() - 1];
+pub const fn trim_whitespace(mut data: &[u8]) -> &[u8] {
+    while let [b' ' | b'\t' | b'\n', remainder @ ..] = data {
+        data = remainder;
+    }
+    while let [remainder @ .., b' ' | b'\t' | b'\n'] = data {
+        data = remainder;
     }
     data
 }