Merge bitcoin/bitcoin#27927: util: Allow std::byte and char Span serialization

fa38d86235 Use only Span{} constructor for byte-like types where possible (MarcoFalke)
fa257bc831 util: Allow std::byte and char Span serialization (MarcoFalke)

Pull request description:

  Seems odd to require developers to cast all byte-like spans passed to serialization to `unsigned char`-spans. Fix that by passing and accepting byte-like spans as-is. Finally, add tests and update the code to use just `Span` where possible.

ACKs for top commit:
  sipa:
    utACK fa38d86235
  achow101:
    ACK fa38d86235
  ryanofsky:
    Code review ACK fa38d86235. This looks great. The second commit really removes a lot of boilerplate and shows why the first commit is useful.

Tree-SHA512: 788592d9ff515c3ebe73d48f9ecbb8d239f5b985af86f09974e508cafb0ca6d73a959350295246b4dfb496149bc56330a0b5d659fc434ba6723dbaba0b7a49e5

src/bench/load_external.cpp

@@ -34,7 +34,7 @@ static void LoadExternalBlockFile(benchmark::Bench& bench)
     ss << static_cast<uint32_t>(benchmark::data::block413567.size());
     // We can't use the streaming serialization (ss << benchmark::data::block413567)
     // because that first writes a compact size.
-    ss.write(MakeByteSpan(benchmark::data::block413567));
+    ss << Span{benchmark::data::block413567};
 
     // Create the test file.
     {