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/PayChan.h

@@ -34,7 +34,7 @@ serializePayChanAuthorization(
     XRPAmount const& amt)
 {
     msg.add32(HashPrefix::paymentChannelClaim);
-    msg.add256(key);
+    msg.addBitString(key);
     msg.add64(amt.drops());
 }