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/nodestore/impl/Shard.cpp

@@ -594,7 +594,7 @@ Shard::finalize(const bool writeSQLite)
     s.add32(version);
     s.add32(firstSeq_);
     s.add32(lastSeq_);
-    s.add256(lastLedgerHash);
+    s.addBitString(lastLedgerHash);
     auto nObj{
         NodeObject::createObject(hotUNKNOWN, std::move(s.modData()), finalKey)};
     try