refactor: Add LIFETIMEBOUND to all (w)txid getters

Then, use the compiler warnings to create copies only where needed.

Also, fix iwyu includes while touching the includes.

src/util/transaction_identifier.h

@@ -1,6 +1,7 @@
 #ifndef BITCOIN_UTIL_TRANSACTION_IDENTIFIER_H
 #define BITCOIN_UTIL_TRANSACTION_IDENTIFIER_H
 
+#include <attributes.h>
 #include <uint256.h>
 #include <util/types.h>
 
@@ -35,7 +36,7 @@ public:
     template <typename Other>
     bool operator<(const Other& other) const { return Compare(other) < 0; }
 
-    uint256 ToUint256() const { return m_wrapped; }
+    const uint256& ToUint256() const LIFETIMEBOUND { return m_wrapped; }
     static transaction_identifier FromUint256(const uint256& id) { return {id}; }
 
     /** Wrapped `uint256` methods. */
@@ -56,7 +57,7 @@ public:
      * TODO: This should be removed once the majority of the code has switched
      * to using the Txid and Wtxid types. Until then it makes for a smoother
      * transition to allow this conversion. */
-    operator uint256() const { return m_wrapped; }
+    operator const uint256&() const LIFETIMEBOUND { return m_wrapped; }
 };
 
 /** Txid commits to all transaction fields except the witness. */