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/DatabaseShardImp.cpp

@@ -778,7 +778,7 @@ DatabaseShardImp::import(Database& source)
                 s.add32(Shard::version);
                 s.add32(firstLedgerSeq(shardIndex));
                 s.add32(lastLedgerSeq(shardIndex));
-                s.add256(*lastLedgerHash);
+                s.addBitString(*lastLedgerHash);
                 auto nObj{NodeObject::createObject(
                     hotUNKNOWN, std::move(s.modData()), Shard::finalKey)};