Merge bitcoin/bitcoin#29114: util: Faster std::byte (pre)vector (un)serialize

fab4169 Allow int8_t optimized vector serialization (MarcoFalke)
facaa14 Faster std::byte (pre)vector (un)serialize (MarcoFalke)

Pull request description:

  Currently, large vectors of `std::byte` are (un)serialized byte-by-byte, which is slow. Fix this, by enabling the already existing optimization for them.

  On my system this gives a 10x speedup for `./src/bench/bench_bitcoin --filter=PrevectorDeserializeTrivial`, when `std::byte` are used:

  ```diff
  diff --git a/src/bench/prevector.cpp b/src/bench/prevector.cpp
  index 2524e21..76b16bc34e 100644
  --- a/src/bench/prevector.cpp
  +++ b/src/bench/prevector.cpp
  @@ -17,7 +17,7 @@ struct nontrivial_t {
   static_assert(!std::is_trivially_default_constructible<nontrivial_t>::value,
                 "expected nontrivial_t to not be trivially constructible");

  -typedef unsigned char trivial_t;
  +typedef std::byte trivial_t;
   static_assert(std::is_trivially_default_constructible<trivial_t>::value,
                 "expected trivial_t to be trivially constructible");

  ```

  However, the optimization does not cover `signed char`. Fix that as well.

ACKs for top commit:
  sipa:
    utACK fab4169
  achow101:
    ACK fab4169
  TheCharlatan:
    ACK fab4169

Tree-SHA512: a3e20f375fd1d0e0dedb827a8ce528de1173ea69660c8c891ad1343a86b422072f6505096fca0d3f8af4442fbe1378a02e32d5974919d4e88ff06934d0258cba

Author: achow101

src/serialize.h

@@ -710,14 +710,12 @@ 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> inline void Serialize(Stream& os, const prevector<N, T>& v);
 template<typename Stream, unsigned int N, typename T> inline void Unserialize(Stream& is, prevector<N, T>& v);
 
 /**
  * vector
- * vectors of unsigned char are a special case and are intended to be serialized as a single opaque blob.
  */
 template<typename Stream, typename T, typename A> inline void Serialize(Stream& os, const std::vector<T, A>& v);
 template<typename Stream, typename T, typename A> inline void Unserialize(Stream& is, std::vector<T, A>& v);
@@ -820,10 +818,9 @@ void Unserialize(Stream& is, std::basic_string<C>& str)
 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>) {
+    if constexpr (BasicByte<T>) { // Use optimized version for unformatted basic bytes
         WriteCompactSize(os, v.size());
-        if (!v.empty())
-            os.write(MakeByteSpan(v));
+        if (!v.empty()) os.write(MakeByteSpan(v));
     } else {
         Serialize(os, Using<VectorFormatter<DefaultFormatter>>(v));
     }
@@ -833,7 +830,7 @@ void Serialize(Stream& os, const prevector<N, T>& v)
 template <typename Stream, unsigned int N, typename T>
 void Unserialize(Stream& is, prevector<N, T>& v)
 {
-    if constexpr (std::is_same_v<T, unsigned char>) {
+    if constexpr (BasicByte<T>) { // Use optimized version for unformatted basic bytes
         // Limit size per read so bogus size value won't cause out of memory
         v.clear();
         unsigned int nSize = ReadCompactSize(is);
@@ -856,10 +853,9 @@ void Unserialize(Stream& is, prevector<N, T>& v)
 template <typename Stream, typename T, typename A>
 void Serialize(Stream& os, const std::vector<T, A>& v)
 {
-    if constexpr (std::is_same_v<T, unsigned char>) {
+    if constexpr (BasicByte<T>) { // Use optimized version for unformatted basic bytes
         WriteCompactSize(os, v.size());
-        if (!v.empty())
-            os.write(MakeByteSpan(v));
+        if (!v.empty()) os.write(MakeByteSpan(v));
     } else if constexpr (std::is_same_v<T, bool>) {
         // A special case for std::vector<bool>, as dereferencing
         // std::vector<bool>::const_iterator does not result in a const bool&
@@ -877,7 +873,7 @@ void Serialize(Stream& os, const std::vector<T, A>& v)
 template <typename Stream, typename T, typename A>
 void Unserialize(Stream& is, std::vector<T, A>& v)
 {
-    if constexpr (std::is_same_v<T, unsigned char>) {
+    if constexpr (BasicByte<T>) { // Use optimized version for unformatted basic bytes
         // Limit size per read so bogus size value won't cause out of memory
         v.clear();
         unsigned int nSize = ReadCompactSize(is);

src/span.h

@@ -287,9 +287,11 @@ Span<std::byte> MakeWritableByteSpan(V&& v) noexcept
 // Helper functions to safely cast basic byte pointers to unsigned char pointers.
 inline unsigned char* UCharCast(char* c) { return reinterpret_cast<unsigned char*>(c); }
 inline unsigned char* UCharCast(unsigned char* c) { return c; }
+inline unsigned char* UCharCast(signed char* c) { return reinterpret_cast<unsigned char*>(c); }
 inline unsigned char* UCharCast(std::byte* c) { return reinterpret_cast<unsigned char*>(c); }
 inline const unsigned char* UCharCast(const char* c) { return reinterpret_cast<const unsigned char*>(c); }
 inline const unsigned char* UCharCast(const unsigned char* c) { return c; }
+inline const unsigned char* UCharCast(const signed char* c) { return reinterpret_cast<const unsigned char*>(c); }
 inline const unsigned char* UCharCast(const std::byte* c) { return reinterpret_cast<const unsigned char*>(c); }
 // Helper concept for the basic byte types.
 template <typename B>