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/entropy.h

@@ -0,0 +1,96 @@
+#include "../ds/address.h"
+
+#include <cstdint>
+#include <random>
+#include <type_traits>
+
+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()
+  {
+    std::random_device rd;
+    uint64_t a = rd();
+    return (a << 32) ^ rd();
+  }
+
+  class LocalEntropy
+  {
+    uint64_t bit_source;
+    uint64_t local_key;
+    uint64_t local_counter;
+    address_t constant_key;
+
+  public:
+    template<typename PAL>
+    void init()
+    {
+      local_key = get_entropy64<PAL>();
+      local_counter = get_entropy64<PAL>();
+      if constexpr (bits::BITS == 64)
+        constant_key = get_next();
+      else
+        constant_key = get_next() & 0xffff'ffff;
+      bit_source = get_next();
+    }
+
+    /**
+     * Returns a bit.
+     *
+     * The bit returned is cycled every 64 calls.
+     * This is a very cheap source of some randomness.
+     * Returns the bottom bit.
+     */
+    uint32_t next_bit()
+    {
+      uint64_t bottom_bit = bit_source & 1;
+      bit_source = (bottom_bit << 63) | (bit_source >> 1);
+      return bottom_bit & 1;
+    }
+
+    /**
+     * A key that is not changed or used to create other keys
+     *
+     * This is for use when there is no storage for the key.
+     */
+    address_t get_constant_key()
+    {
+      return constant_key;
+    }
+
+    /**
+     * Source of random 64bit values
+     *
+     * Has a 2^64 period.
+     *
+     * Applies a Feistel cipher to a counter
+     */
+    uint64_t get_next()
+    {
+      uint64_t c = ++local_counter;
+      uint64_t bottom;
+      for (int i = 0; i < 2; i++)
+      {
+        bottom = c & 0xffff'fffff;
+        c = (c << 32) | (((bottom * local_key) ^ c) >> 32);
+      }
+      return c;
+    }
+
+    /**
+     * Refresh `next_bit` source of bits.
+     *
+     * This loads new entropy into the `next_bit` values.
+     */
+    void refresh_bits()
+    {
+      bit_source = get_next();
+    }
+  };
+} // namespace snmalloc