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/protocol/impl/STAmount.cpp

@@ -624,8 +624,8 @@ STAmount::add(Serializer& s) const
                 (static_cast<std::uint64_t>(mOffset + 512 + 256 + 97)
                  << (64 - 10)));
 
-        s.add160(mIssue.currency);
-        s.add160(mIssue.account);
+        s.addBitString(mIssue.currency);
+        s.addBitString(mIssue.account);
     }
 }