library: Stabilize const_replace

Depends on stabilizing `const_ptr_write`.

Const-stabilizes:
- `core::mem::replace`
- `core::ptr::replace`

library/core/src/lib.rs

@@ -138,7 +138,6 @@
 #![feature(const_ptr_is_null)]
 #![feature(const_ptr_sub_ptr)]
 #![feature(const_raw_ptr_comparison)]
-#![feature(const_replace)]
 #![feature(const_size_of_val)]
 #![feature(const_size_of_val_raw)]
 #![feature(const_strict_overflow_ops)]