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.

1 files changed, 3 insertions, 3 deletions

diff --git a/src/interfaces/wallet.h b/src/interfaces/wallet.h
index e29fefae56..9bd033d991 100644
--- a/src/interfaces/wallet.h
+++ b/src/interfaces/wallet.h
@@ -88,7 +88,7 @@ public:
     virtual std::string getWalletName() = 0;
 
     // Get a new address.
-    virtual BResult<CTxDestination> getNewDestination(const OutputType type, const std::string label) = 0;
+    virtual util::Result<CTxDestination> getNewDestination(const OutputType type, const std::string label) = 0;
 
     //! Get public key.
     virtual bool getPubKey(const CScript& script, const CKeyID& address, CPubKey& pub_key) = 0;
@@ -139,7 +139,7 @@ public:
     virtual void listLockedCoins(std::vector<COutPoint>& outputs) = 0;
 
     //! Create transaction.
-    virtual BResult<CTransactionRef> createTransaction(const std::vector<wallet::CRecipient>& recipients,
+    virtual util::Result<CTransactionRef> createTransaction(const std::vector<wallet::CRecipient>& recipients,
         const wallet::CCoinControl& coin_control,
         bool sign,
         int& change_pos,
@@ -329,7 +329,7 @@ public:
    virtual std::string getWalletDir() = 0;
 
    //! Restore backup wallet
-   virtual BResult<std::unique_ptr<Wallet>> restoreWallet(const fs::path& backup_file, const std::string& wallet_name, std::vector<bilingual_str>& warnings) = 0;
+   virtual util::Result<std::unique_ptr<Wallet>> restoreWallet(const fs::path& backup_file, const std::string& wallet_name, std::vector<bilingual_str>& warnings) = 0;
 
    //! Return available wallets in wallet directory.
    virtual std::vector<std::string> listWalletDir() = 0;