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/digest.h

@@ -208,7 +208,7 @@ private:
 
     inline void erase(std::true_type)
     {
-        beast::secure_erase(&h_, sizeof(h_));
+        secure_erase(&h_, sizeof(h_));
     }
 };
 
@@ -221,16 +221,6 @@ using sha512_half_hasher_s = detail::basic_sha512_half_hasher<true>;
 
 //------------------------------------------------------------------------------
 
-#ifdef _MSC_VER
-// Call from main to fix magic statics pre-VS2015
-inline void
-sha512_deprecatedMSVCWorkaround()
-{
-    beast::sha512_hasher h;
-    auto const digest = static_cast<beast::sha512_hasher::result_type>(h);
-}
-#endif
-
 /** Returns the SHA512-Half of a series of objects. */
 template <class... Args>
 sha512_half_hasher::result_type