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/alloc.h

@@ -999,6 +999,15 @@ namespace snmalloc
               (void*)((size_t)super + OS_PAGE_SIZE),
               SUPERSLAB_SIZE - OS_PAGE_SIZE);
           }
+          else if constexpr (decommit_strategy == DecommitSuperLazy)
+          {
+            static_assert(
+              std::remove_reference_t<decltype(
+                large_allocator
+                  .memory_provider)>::supports_low_memory_notification,
+              "A lazy decommit strategy cannot be implemented on platforms "
+              "without low memory notifications");
+          }
 
           pagemap().clear_slab(super);
           large_allocator.dealloc(super, 0);