Update library functions with stability attributes

This may not be strictly minimal, but all unstable functions also need a
`rustc_const_unstable` attribute.

library/core/src/lib.rs

@@ -75,11 +75,13 @@
 #![feature(const_float_bits_conv)]
 #![feature(const_overflowing_int_methods)]
 #![feature(const_int_unchecked_arith)]
+#![feature(const_mut_refs)]
 #![feature(const_int_pow)]
 #![feature(constctlz)]
 #![feature(const_panic)]
 #![feature(const_pin)]
 #![feature(const_fn_union)]
+#![feature(const_fn)]
 #![feature(const_generics)]
 #![feature(const_option)]
 #![feature(const_precise_live_drops)]