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/Sign.h

@@ -84,7 +84,7 @@ startMultiSigningData(STObject const& obj);
 inline void
 finishMultiSigningData(AccountID const& signingID, Serializer& s)
 {
-    s.add160(signingID);
+    s.addBitString(signingID);
 }
 
 }  // namespace ripple