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/overlay/impl/Cluster.cpp

@@ -63,9 +63,7 @@ Cluster::update(
 {
     std::lock_guard lock(mutex_);
 
-    // We can't use auto here yet due to the libstdc++ issue
-    // described at https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68190
-    std::set<ClusterNode, Comparator>::iterator iter = nodes_.find(identity);
+    auto iter = nodes_.find(identity);
 
     if (iter != nodes_.end())
     {