Initial cut at a lazy decommit strategy.

This does not deallocate memory until the OS tells us that we are short
on memory, then tries to decommit all of the cached chunks (except for
the first page, used for the linked lists).

Nowhere near enough testing to commit to master yet!

src/mem/baseslab.h

@@ -10,7 +10,12 @@ namespace snmalloc
     Fresh = 0,
     Large,
     Medium,
-    Super
+    Super,
+    /**
+     * If the decommit policy is lazy, slabs are moved to this state when all
+     * pages other than the first one have been decommitted.
+     */
+    Decommitted
   };
 
   class Baseslab