Make Secondary Allocator a template parameter (#774)

This refactors the use of the Secondary Allocator, so that out of tree implementations can be used.

src/test/func/domestication/domestication.cc

@@ -5,6 +5,7 @@
 #include <snmalloc/backend/backend.h>
 #include <snmalloc/backend/standard_range.h>
 #include <snmalloc/backend_helpers/backend_helpers.h>
+#include <snmalloc/mem/secondary/default.h>
 #include <snmalloc/snmalloc_core.h>
 
 // Specify type of allocator
@@ -18,6 +19,7 @@ namespace snmalloc
     using Pal = DefaultPal;
     using PagemapEntry = DefaultPagemapEntry<NoClientMetaDataProvider>;
     using ClientMeta = NoClientMetaDataProvider;
+    using SecondaryAllocator = DefaultSecondaryAllocator;
 
   private:
     using ConcretePagemap =

src/test/func/fixed_region/fixed_region.cc

@@ -1,4 +1,3 @@
-#include "snmalloc/mem/secondary.h"
 #include "test/setup.h"
 
 #include <iostream>

src/test/func/fixed_region_alloc/fixed_region_alloc.cc

@@ -1,4 +1,3 @@
-#include "snmalloc/mem/secondary.h"
 #include "test/setup.h"
 
 #include <iostream>