Stabilize the cell_update feature

Included API:

    impl<T: Copy> Cell<T> {
        pub fn update(&self, f: impl FnOnce(T) -> T);
    }

Closes: rust-lang#50186

Author: tgross35

core/src/cell.rs

@@ -549,16 +549,14 @@ impl<T: Copy> Cell<T> {
     /// # Examples
     ///
     /// ```
-    /// #![feature(cell_update)]
-    ///
     /// use std::cell::Cell;
     ///
     /// let c = Cell::new(5);
     /// c.update(|x| x + 1);
     /// assert_eq!(c.get(), 6);
     /// ```
     #[inline]
-    #[unstable(feature = "cell_update", issue = "50186")]
+    #[stable(feature = "cell_update", since = "CURRENT_RUSTC_VERSION")]
     pub fn update(&self, f: impl FnOnce(T) -> T) {
         let old = self.get();
         self.set(f(old));

coretests/tests/lib.rs

@@ -12,7 +12,6 @@
 #![feature(async_iterator)]
 #![feature(bigint_helper_methods)]
 #![feature(bstr)]
-#![feature(cell_update)]
 #![feature(char_max_len)]
 #![feature(clone_to_uninit)]
 #![feature(const_eval_select)]