Use spans of std::byte in serialize

This switches .read() and .write() to take spans of bytes.

src/bench/rpc_blockchain.cpp

@@ -23,8 +23,8 @@ struct TestBlockAndIndex {
     TestBlockAndIndex()
     {
         CDataStream stream(benchmark::data::block413567, SER_NETWORK, PROTOCOL_VERSION);
-        char a = '\0';
-        stream.write(&a, 1); // Prevent compaction
+        std::byte a{0};
+        stream.write({&a, 1}); // Prevent compaction
 
         stream >> block;