Stabilize the cell_update feature

Included API:

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

Closes: rust-lang/rust#50186

Author: tgross35

src/lib.rs

@@ -1,6 +1,5 @@
 #![feature(rustc_private)]
 #![feature(cfg_match)]
-#![feature(cell_update)]
 #![feature(float_gamma)]
 #![feature(float_erf)]
 #![feature(map_try_insert)]