Merge bitcoin/bitcoin#34483: refactor: Use SpanReader over DataStream

fa0677d131 refactor: Use SpanReader over DataStream (MarcoFalke)
fad3eb3956 refactor: Use SpanReader over DataStream (MarcoFalke)
fa06e26764 refactor: [qt] Use SpanReader to avoid two vector copies (MarcoFalke)
fabd4d2e2e refactor: Avoid UB in SpanReader::ignore (MarcoFalke)
fa20bc2ec2 refactor: Use empty() over eof() in the streams interface (MarcoFalke)
fa879db735 test: Read debug log for self-checking comment (MarcoFalke)

Pull request description:

  This changes all places, where possible, to use SpanReader over DataStream. This makes the code easier to read and reason about, because `SpanReader` can never write data. Also, the code should be minimally faster, because it avoids a full redundant copy of the whole vector of bytes.

ACKs for top commit:
  stickies-v:
    re-ACK fa0677d131
  achow101:
    ACK fa0677d131
  janb84:
    re ACK fa0677d131
  sipa:
    crACK fa0677d131

Tree-SHA512: 1d9f43fc6e71d481cf7b8f8457f479745ee331734649e9e2c2ab00ce5d317112796c77afc328612ed004e65ac5c16fc92279d760cfb012cfddce9098c4af810f

test/functional/p2p_segwit.py

@@ -1198,8 +1198,8 @@ class SegWitTest(BitcoinTestFramework):
         block.vtx = [block.vtx[0]]
         self.update_witness_block_with_transactions(block, [tx2])
         # This block doesn't result in a specific reject reason, but an iostream exception:
-        # "Exception 'CDataStream::read(): end of data: unspecified iostream_category error' (...) caught"
-        test_witness_block(self.nodes[0], self.test_node, block, accepted=False)
+        with self.nodes[0].assert_debug_log(["Exception 'DataStream::read(): end of data"]):
+            test_witness_block(self.nodes[0], self.test_node, block, accepted=False)
 
         # Now make one of the intermediate witnesses be incorrect
         tx2.wit.vtxinwit.append(CTxInWitness())