change split_at_checked to use split_at_unchecked internally

Author: KGrewal1

library/core/src/str/mod.rs

@@ -720,7 +720,7 @@ impl str {
         // is_char_boundary checks that the index is in [0, .len()]
         if self.is_char_boundary(mid) {
             // SAFETY: just checked that `mid` is on a char boundary.
-            Some(unsafe { (self.get_unchecked(0..mid), self.get_unchecked(mid..self.len())) })
+           Some(unsafe { self.split_at_unchecked(mid) })
         } else {
             None
         }