core: make str::split_at_unchecked() inline

library/core/src/str/mod.rs

@@ -952,6 +952,7 @@ 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.
+    #[inline]
     const unsafe fn split_at_unchecked(&self, mid: usize) -> (&str, &str) {
         let len = self.len();
         let ptr = self.as_ptr();