Merge bitcoin/bitcoin#34032: util: Add some more Unexpected and Expected methods

faa59b3679 util: Add Expected::swap() (MarcoFalke)
fabb47e4e3 util: Implement Expected::operator*()&& (MarcoFalke)
fab9721430 util: Implement Expected::value()&& and Expected::error()&& (MarcoFalke)
fac4800959 util: Add Expected<void, E> specialization (MarcoFalke)
fa6575d6c2 util: Make Expected::value() throw (MarcoFalke)
fa1de1103f util: Add Unexpected::error() (MarcoFalke)
faa109f8be test: refactor: Use BOOST_CHECK_EQUAL over BOOST_CHECK == (MarcoFalke)
fad4a9fe2b Set bugprone-unused-return-value.AllowCastToVoid (MarcoFalke)

Pull request description:

  Reviewers requested more member functions In https://github.com/bitcoin/bitcoin/pull/34006.

  They are currently unused, but bring the port closer to the original `std::expected` implementation:

  * Make `Expected::value()` throw when no value exists
  * Add `Unexpected::error()` methods
  * Add `Expected<void, E>` specialization
  * Add `Expected::value()&&` and `Expected::error()&&` methods
  * Add `Expected::swap()`

  Also, include a tiny tidy fixup:

  * tidy: Set `AllowCastToVoid` in the `bugprone-unused-return-value` check

ACKs for top commit:
  stickies-v:
    re-ACK faa59b3679
  ryanofsky:
    Code review ACK faa59b3679. Thanks for the update. The commit I objected to is fixed now and the rest of the implementation seems good enough for code that's probably temporary.
  hodlinator:
    re-ACK faa59b3679

Tree-SHA512: b6ac28c1e7241837d9db83fe7534d713ca1283c20a77d2273743157d329f041ec0b503658d14b2f4425211808b61a88fed115d77149e0546825acd3bd9198edf

src/httpserver.cpp

@@ -330,7 +330,7 @@ static void http_request_cb(struct evhttp_request* req, void* arg)
         std::unique_ptr<HTTPWorkItem> item(new HTTPWorkItem(std::move(hreq), path, i->handler));
         assert(g_work_queue);
         if (g_work_queue->Enqueue(item.get())) {
-            [[maybe_unused]] auto _{item.release()}; /* if true, queue took ownership */
+            (void)item.release(); /* if true, queue took ownership */
         } else {
             LogWarning("Request rejected because http work queue depth exceeded, it can be increased with the -rpcworkqueue= setting");
             item->req->WriteReply(HTTP_SERVICE_UNAVAILABLE, "Work queue depth exceeded");