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/psbt.cpp

@@ -617,7 +617,7 @@ bool DecodeBase64PSBT(PartiallySignedTransaction& psbt, const std::string& base6
 
 bool DecodeRawPSBT(PartiallySignedTransaction& psbt, std::span<const std::byte> tx_data, std::string& error)
 {
-    DataStream ss_data{tx_data};
+    SpanReader ss_data{tx_data};
     try {
         ss_data >> psbt;
         if (!ss_data.empty()) {