make split_at_mut_unchecked const

Author: KGrewal1

library/core/src/str/mod.rs

@@ -772,7 +772,7 @@ impl str {
     ///
     /// The caller must ensure that `mid` is a valid byte offset from the start
     /// of the string and falls on the boundary of a UTF-8 code point.
-    unsafe fn split_at_mut_unchecked(&mut self, mid: usize) -> (&mut str, &mut str) {
+    unsafe const fn split_at_mut_unchecked(&mut self, mid: usize) -> (&mut str, &mut str) {
         let len = self.len();
         let ptr = self.as_mut_ptr();
         // SAFETY: caller guarantees `mid` is on a char boundary.