Simplify HashPrefix by converting to an enum

src/ripple/protocol/impl/Serializer.cpp

@@ -21,6 +21,7 @@
 #include <ripple/basics/Log.h>
 #include <ripple/protocol/digest.h>
 #include <ripple/protocol/Serializer.h>
+#include <type_traits>
 
 namespace ripple {
 
@@ -52,6 +53,17 @@ int Serializer::add32 (std::uint32_t i)
     return ret;
 }
 
+
+int Serializer::add32 (HashPrefix p)
+{
+    // This should never trigger; the size & type of a hash prefix are
+    // integral parts of the protocol and unlikely to ever change.
+    static_assert(std::is_same_v<std::uint32_t,
+        std::underlying_type_t<decltype(p)>>);
+
+    return add32(safe_cast<std::uint32_t>(p));
+}
+
 int Serializer::add64 (std::uint64_t i)
 {
     int ret = mData.size ();