refactor: Replace BResult with util::Result

Rename `BResult` class to `util::Result` and update the class interface to be
more compatible with `std::optional` and with a full-featured result class
implemented in https://github.com/bitcoin/bitcoin/pull/25665. Motivation for
this change is to update existing `BResult` usages now so they don't have to
change later when more features are added in #25665.

This change makes the following improvements originally implemented in #25665:

- More explicit API. Drops potentially misleading `BResult` constructor that
  treats any bilingual string argument as an error. Adds `util::Error`
  constructor so it is never ambiguous when a result is being assigned an error
  or non-error value.

- Better type compatibility. Supports `util::Result<bilingual_str>` return
  values to hold translated messages which are not errors.

- More standard and consistent API. `util::Result` supports most of the same
  operators and methods as `std::optional`. `BResult` had a less familiar
  interface with `HasRes`/`GetObj`/`ReleaseObj` methods. The Result/Res/Obj
  naming was also not internally consistent.

- Better code organization. Puts `src/util/` code in the `util::` namespace so
  naming reflects code organization and it is obvious where the class is coming
  from. Drops "B" from name because it is undocumented what it stands for
  (bilingual?)

- Has unit tests.

src/qt/addresstablemodel.cpp

@@ -384,7 +384,7 @@ QString AddressTableModel::addRow(const QString &type, const QString &label, con
                 return QString();
             }
         }
-        strAddress = EncodeDestination(op_dest.GetObj());
+        strAddress = EncodeDestination(*op_dest);
     }
     else
     {

src/qt/walletcontroller.cpp

@@ -393,8 +393,8 @@ void RestoreWalletActivity::restore(const fs::path& backup_file, const std::stri
     QTimer::singleShot(0, worker(), [this, backup_file, wallet_name] {
         auto wallet{node().walletLoader().restoreWallet(backup_file, wallet_name, m_warning_message)};
 
-        m_error_message = wallet ? bilingual_str{} : wallet.GetError();
-        if (wallet) m_wallet_model = m_wallet_controller->getOrCreateWallet(wallet.ReleaseObj());
+        m_error_message = util::ErrorString(wallet);
+        if (wallet) m_wallet_model = m_wallet_controller->getOrCreateWallet(std::move(*wallet));
 
         QTimer::singleShot(0, this, &RestoreWalletActivity::finish);
     });

src/qt/walletmodel.cpp

@@ -207,7 +207,7 @@ WalletModel::SendCoinsReturn WalletModel::prepareTransaction(WalletModelTransact
 
         auto& newTx = transaction.getWtx();
         const auto& res = m_wallet->createTransaction(vecSend, coinControl, !wallet().privateKeysDisabled() /* sign */, nChangePosRet, nFeeRequired);
-        newTx = res ? res.GetObj() : nullptr;
+        newTx = res ? *res : nullptr;
         transaction.setTransactionFee(nFeeRequired);
         if (fSubtractFeeFromAmount && newTx)
             transaction.reassignAmounts(nChangePosRet);
@@ -218,7 +218,7 @@ WalletModel::SendCoinsReturn WalletModel::prepareTransaction(WalletModelTransact
             {
                 return SendCoinsReturn(AmountWithFeeExceedsBalance);
             }
-            Q_EMIT message(tr("Send Coins"), QString::fromStdString(res.GetError().translated),
+            Q_EMIT message(tr("Send Coins"), QString::fromStdString(util::ErrorString(res).translated),
                 CClientUIInterface::MSG_ERROR);
             return TransactionCreationFailed;
         }