Cleaner implementation of signed pointers. (#347)

* Cleaner implementation of signed pointers.

This encodes a back pointer in each node.  The back pointer is stored
in an encoded form so that it is hard to corrupt and trick the allocator
into following incorrect pointers.

This changes the encoding from previously being a Feistel network on
the next pointer that was using the prev as part of the key, to now
effectively using a doubly linked queue, where the back pointers are
scrambled, so it is hard to forge them.

This has the positive effects of
 - Not needing to store previous while building the list, as the append
   nows, curr and next at the point of writing into next, and does not
   need an additional previous.
 - The encoding is not affecting the actual next value, so more
   instructions can be executed in parallel by the CPU.

Future extensions, store a changing key in the FreeListBuilder so it
becomes harder to try to forge the previous token.

This approach can also be applied to the remote list, and will in a
subsequent PR.  This enables the idea to be tested.

* Remove unused header.

* Apply suggestions from code review

Co-authored-by: Nathaniel Wesley Filardo <VP331RHQ115POU58JFRLKB7OPA0L18E3@cmx.ietfng.org>

Co-authored-by: Nathaniel Wesley Filardo <VP331RHQ115POU58JFRLKB7OPA0L18E3@cmx.ietfng.org>

src/mem/corealloc.h

@@ -9,8 +9,6 @@
 #include "sizeclasstable.h"
 #include "slaballocator.h"
 
-#include <iostream>
-
 namespace snmalloc
 {
   template<typename SharedStateHandle>