Expose Entropy

Define various parts of random that can be used to make the layout of
memory more random.  Thread this through the allocator.

Expose the concept as part of the Pal. Subsequent commits will expose
that on different platforms.

src/mem/chunkmap.h

@@ -72,7 +72,7 @@ namespace snmalloc
  *     internal tree node in the non-flat pagemap).
  */
 #  define SNMALLOC_MAX_FLATPAGEMAP_SIZE \
-    (pal_supports<LazyCommit> ? 256ULL * 1024 * 1024 : PAGEMAP_NODE_SIZE)
+    (pal_supports<LazyCommit, Pal> ? 256ULL * 1024 * 1024 : PAGEMAP_NODE_SIZE)
 #endif
   static constexpr bool CHUNKMAP_USE_FLATPAGEMAP =
     SNMALLOC_MAX_FLATPAGEMAP_SIZE >=