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/STObject.h

@@ -239,8 +239,6 @@ private:
         }
     };
 
-    enum { reserveSize = 20 };
-
     using list_type = std::vector<detail::STVar>;
 
     list_type v_;
@@ -281,7 +279,7 @@ public:
 
     explicit STObject(SField const& name);
 
-    virtual ~STObject();
+    virtual ~STObject() = default;
 
     STBase*
     copy(std::size_t n, void* buf) const override