refactor: Use SpanReader over DataStream

This refactor does not change behavior. However, it avoids a vector
copy, which can lead to a minimal speed-up of 1%-5%, depending on the
call-site. This is mostly relevant for the fuzz tests and utils that
read large blobs of data (like a full block).

src/rest.cpp

@@ -451,8 +451,7 @@ static bool rest_block(const std::any& context,
     case RESTResponseFormat::JSON: {
         if (tx_verbosity) {
             CBlock block{};
-            DataStream block_stream{*block_data};
-            block_stream >> TX_WITH_WITNESS(block);
+            SpanReader{*block_data} >> TX_WITH_WITNESS(block);
             UniValue objBlock = blockToJSON(chainman.m_blockman, block, *tip, *pblockindex, *tx_verbosity, chainman.GetConsensus().powLimit);
             std::string strJSON = objBlock.write() + "\n";
             req->WriteHeader("Content-Type", "application/json");