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/pal/pal.h

@@ -66,12 +66,6 @@ namespace snmalloc
     Pal::error(str);
   }
 
-  /**
-   * Query whether the PAL supports a specific feature.
-   */
-  template<PalFeatures F, SNMALLOC_CONCEPT(ConceptPAL) PAL = Pal>
-  constexpr static bool pal_supports = (PAL::pal_features & F) == F;
-
   // Used to keep Superslab metadata committed.
   static constexpr size_t OS_PAGE_SIZE = Pal::page_size;
 

src/pal/pal_consts.h

@@ -41,6 +41,10 @@ namespace snmalloc
      * should be pre-allocated.
      */
     NoAllocation = (1 << 3),
+    /**
+     * This Pal provides a source of Entropy
+     */
+    Entropy = (1 << 4),
   };
   /**
    * Flag indicating whether requested memory should be zeroed.
@@ -125,4 +129,10 @@ namespace snmalloc
       }
     }
   };
+
+  /**
+   * Query whether the PAL supports a specific feature.
+   */
+  template<PalFeatures F, typename PAL>
+  constexpr static bool pal_supports = (PAL::pal_features & F) == F;
 } // namespace snmalloc