Prevent passing of non-POD types to POD-only interfaces:

This tidies up the code that produces random numbers to conform
to programming best practices and reduce dependencies.

* Use std::random_device instead of platform-specific code
* Remove RandomNumbers class and use free functions instead

src/ripple/protocol/impl/RippleAddress.cpp

@@ -867,7 +867,7 @@ void RippleAddress::setSeedRandom ()
     // XXX Maybe we should call MakeNewKey
     uint128 key;
 
-    RandomNumbers::getInstance ().fillBytes (key.begin (), key.size ());
+    random_fill (key.begin (), key.size ());
 
     RippleAddress::setSeed (key);
 }