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/overlay/impl/OverlayImpl.cpp

@@ -237,7 +237,7 @@ OverlayImpl::onHandoff(
         return handoff;
     }
 
-    // TODO Validate HTTP request
+    // Validate HTTP request
 
     {
         auto const types = beast::rfc2616::split_commas(request["Connect-As"]);
@@ -1083,14 +1083,13 @@ OverlayImpl::processValidatorList(
 {
     // If the target is in the form "/vl/<validator_list_public_key>",
     // return the most recent validator list for that key.
-    if (!req.target().starts_with("/vl/") || !setup_.vlEnabled)
+    constexpr std::string_view prefix("/vl/");
+
+    if (!req.target().starts_with(prefix.data()) || !setup_.vlEnabled)
         return false;
 
-    auto key = req.target();
-    if (key.starts_with("/vl/"))
-        key.remove_prefix(strlen("/vl/"));
-    else
-        key.remove_prefix(strlen("/unl/"));
+    auto key = req.target().substr(prefix.size());
+
     if (key.empty())
         return false;