Add a helper Impl() function for CRTP base 'Hash'

Chaos/Hashing/Hash.hpp

@@ -12,12 +12,23 @@ class Hash
 public:
     auto GetRawDigest() const
     {
-        return static_cast<const T &>(*this).GetRawDigest();
+        return Impl().GetRawDigest();
     }
 
     std::string ToHexString() const
     {
-        return static_cast<const T &>(*this).ToHexString();
+        return Impl().ToHexString();
+    }
+
+private:
+    const T & Impl() const
+    {
+        return static_cast<const T &>(*this);
+    }
+
+    T & Impl()
+    {
+        return static_cast<T &>(*this);
     }
 };