Clean up and modernize code:

This commit removes obsolete comments, dead or no longer useful
code, and workarounds for several issues that were present in older
compilers that we no longer support.

Specifically:

- It improves the transaction metadata handling class, simplifying
  its use and making it less error-prone.
- It reduces the footprint of the Serializer class by consolidating
  code and leveraging templates.
- It cleanups the ST* class hierarchy, removing dead code, improving
  and consolidating code to reduce complexity and code duplication.
- It shores up the handling of currency codes and the conversation
  between 160-bit currency codes and their string representation.
- It migrates beast::secure_erase to the ripple namespace and uses
  a call to OpenSSL_cleanse instead of the custom implementation.

src/ripple/app/main/Main.cpp

@@ -779,14 +779,11 @@ main(int argc, char** argv)
         _ftime(&t);
 #endif
     }
-    ripple::sha512_deprecatedMSVCWorkaround();
 #endif
 
     atexit(&google::protobuf::ShutdownProtobufLibrary);
 
     auto const result(ripple::run(argc, argv));
 
-    beast::basic_seconds_clock_main_hook();
-
     return result;
 }