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/test/fuzz/tx_in.cpp

@@ -13,10 +13,9 @@
 
 FUZZ_TARGET(tx_in)
 {
-    DataStream ds{buffer};
     CTxIn tx_in;
     try {
-        ds >> tx_in;
+        SpanReader{buffer} >> tx_in;
     } catch (const std::ios_base::failure&) {
         return;
     }