const_deallocate: Don't deallocate memory allocated in an another const. Does nothing at runtime.

`const_allocate`:  Returns a null pointer at runtime.

library/core/tests/lib.rs

@@ -13,6 +13,7 @@
 #![feature(const_bool_to_option)]
 #![feature(const_cell_into_inner)]
 #![feature(const_convert)]
+#![feature(const_heap)]
 #![feature(const_maybe_uninit_as_mut_ptr)]
 #![feature(const_maybe_uninit_assume_init)]
 #![feature(const_maybe_uninit_assume_init_read)]