NFC: move get_entropy64 from mem to ds

These functions depend only on the PAL and so can live lower down the
stack.

src/snmalloc/ds/ds.h

@@ -6,6 +6,7 @@
 #include "../pal/pal.h"
 #include "aba.h"
 #include "allocconfig.h"
+#include "entropy.h"
 #include "flaglock.h"
 #include "mpmcstack.h"
 #include "singleton.h"

src/snmalloc/ds/entropy.h

@@ -0,0 +1,26 @@
+#pragma once
+
+#ifndef SNMALLOC_PLATFORM_HAS_GETENTROPY
+#  include <random>
+#endif
+
+namespace snmalloc
+{
+  template<typename PAL>
+  std::enable_if_t<pal_supports<Entropy, PAL>, uint64_t> get_entropy64()
+  {
+    return PAL::get_entropy64();
+  }
+
+  template<typename PAL>
+  std::enable_if_t<!pal_supports<Entropy, PAL>, uint64_t> get_entropy64()
+  {
+#ifdef SNMALLOC_PLATFORM_HAS_GETENTROPY
+    return DefaultPal::get_entropy64();
+#else
+    std::random_device rd;
+    uint64_t a = rd();
+    return (a << 32) ^ rd();
+#endif
+  }
+} // namespace snmalloc

src/snmalloc/mem/entropy.h

@@ -1,33 +1,13 @@
 #pragma once
 
+#include "../ds/ds.h"
 #include "../pal/pal.h"
 
 #include <cstdint>
 #include <type_traits>
-#ifndef SNMALLOC_PLATFORM_HAS_GETENTROPY
-#  include <random>
-#endif
 
 namespace snmalloc
 {
-  template<typename PAL>
-  std::enable_if_t<pal_supports<Entropy, PAL>, uint64_t> get_entropy64()
-  {
-    return PAL::get_entropy64();
-  }
-
-  template<typename PAL>
-  std::enable_if_t<!pal_supports<Entropy, PAL>, uint64_t> get_entropy64()
-  {
-#ifdef SNMALLOC_PLATFORM_HAS_GETENTROPY
-    return DefaultPal::get_entropy64();
-#else
-    std::random_device rd;
-    uint64_t a = rd();
-    return (a << 32) ^ rd();
-#endif
-  }
-
   struct FreeListKey
   {
     address_t key1;