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/consensus/RCLValidations.cpp

@@ -179,7 +179,7 @@ handleNewValidation(
         s << (val->isTrusted() ? "trusted" : "untrusted") << " "
           << (val->isFull() ? "full" : "partial") << " validation: " << hash
           << " from " << id << " via " << source << ": " << msg << "\n"
-          << " [" << val->getSerializer().getHex() << "]";
+          << " [" << val->getSerializer().slice() << "]";
     };
 
     if (!val->isFieldPresent(sfLedgerSequence))