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.

Builds/CMake/RippledCore.cmake

@@ -114,7 +114,8 @@ target_sources (xrpl_core PRIVATE
   src/ripple/crypto/impl/RFC1751.cpp
   src/ripple/crypto/impl/csprng.cpp
   src/ripple/crypto/impl/ec_key.cpp
-  src/ripple/crypto/impl/openssl.cpp)
+  src/ripple/crypto/impl/openssl.cpp
+  src/ripple/crypto/impl/secure_erase.cpp)
 
 add_library (Ripple::xrpl_core ALIAS xrpl_core)
 target_include_directories (xrpl_core
@@ -168,6 +169,7 @@ install (
     src/ripple/crypto/GenerateDeterministicKey.h
     src/ripple/crypto/RFC1751.h
     src/ripple/crypto/csprng.h
+    src/ripple/crypto/secure_erase.h
   DESTINATION include/ripple/crypto)
 install (
   FILES
@@ -267,7 +269,6 @@ install (
     src/ripple/beast/crypto/detail/ripemd_context.h
     src/ripple/beast/crypto/detail/sha2_context.h
     src/ripple/beast/crypto/ripemd.h
-    src/ripple/beast/crypto/secure_erase.h
     src/ripple/beast/crypto/sha2.h
   DESTINATION include/ripple/beast/crypto)
 install (