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.
