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/STArray.cpp

@@ -24,14 +24,6 @@
 
 namespace ripple {
 
-STArray::STArray()
-{
-    // VFALCO NOTE We need to determine if this is
-    //             the right thing to do, and consider
-    //             making it optional.
-    // v_.reserve(reserveSize);
-}
-
 STArray::STArray(STArray&& other)
     : STBase(other.getFName()), v_(std::move(other.v_))
 {
@@ -52,7 +44,6 @@ STArray::STArray(int n)
 
 STArray::STArray(SField const& f) : STBase(f)
 {
-    v_.reserve(reserveSize);
 }
 
 STArray::STArray(SField const& f, int n) : STBase(f)