refactor: use "if constexpr" in prevector's Serialize()

This gets rid of unnecessarily creating a temporary object T() to call
the right function.

src/serialize.h

@@ -642,8 +642,6 @@ template<typename Stream, typename C> void Unserialize(Stream& is, std::basic_st
  * prevector
  * prevectors of unsigned char are a special case and are intended to be serialized as a single opaque blob.
  */
-template<typename Stream, unsigned int N, typename T> void Serialize_impl(Stream& os, const prevector<N, T>& v, const unsigned char&);
-template<typename Stream, unsigned int N, typename T, typename V> void Serialize_impl(Stream& os, const prevector<N, T>& v, const V&);
 template<typename Stream, unsigned int N, typename T> inline void Serialize(Stream& os, const prevector<N, T>& v);
 template<typename Stream, unsigned int N, typename T> void Unserialize_impl(Stream& is, prevector<N, T>& v, const unsigned char&);
 template<typename Stream, unsigned int N, typename T, typename V> void Unserialize_impl(Stream& is, prevector<N, T>& v, const V&);
@@ -751,24 +749,16 @@ void Unserialize(Stream& is, std::basic_string<C>& str)
 /**
  * prevector
  */
-template<typename Stream, unsigned int N, typename T>
-void Serialize_impl(Stream& os, const prevector<N, T>& v, const unsigned char&)
-{
-    WriteCompactSize(os, v.size());
-    if (!v.empty())
-        os.write(MakeByteSpan(v));
-}
-
-template<typename Stream, unsigned int N, typename T, typename V>
-void Serialize_impl(Stream& os, const prevector<N, T>& v, const V&)
-{
-    Serialize(os, Using<VectorFormatter<DefaultFormatter>>(v));
-}
-
-template<typename Stream, unsigned int N, typename T>
-inline void Serialize(Stream& os, const prevector<N, T>& v)
-{
-    Serialize_impl(os, v, T());
+template <typename Stream, unsigned int N, typename T>
+void Serialize(Stream& os, const prevector<N, T>& v)
+{
+    if constexpr (std::is_same_v<T, unsigned char>) {
+        WriteCompactSize(os, v.size());
+        if (!v.empty())
+            os.write(MakeByteSpan(v));
+    } else {
+        Serialize(os, Using<VectorFormatter<DefaultFormatter>>(v));
+    }
 }