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/STObject.cpp

@@ -26,15 +26,6 @@
 
 namespace ripple {
 
-STObject::~STObject()
-{
-#if 0
-    // Turn this on to get a histogram on exit
-    static Log log;
-    log(v_.size());
-#endif
-}
-
 STObject::STObject(STObject&& other)
     : STBase(other.getFName()), v_(std::move(other.v_)), mType(other.mType)
 {
@@ -42,8 +33,6 @@ STObject::STObject(STObject&& other)
 
 STObject::STObject(SField const& name) : STBase(name), mType(nullptr)
 {
-    // VFALCO TODO See if this is the right thing to do
-    // v_.reserve(reserveSize);
 }
 
 STObject::STObject(SOTemplate const& type, SField const& name) : STBase(name)