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, 5 insertions, 5 deletions

diff --git a/src/wallet/wallet.cpp b/src/wallet/wallet.cpp
index 9a48a1ca21..e750cd5a2c 100644
--- a/src/wallet/wallet.cpp
+++ b/src/wallet/wallet.cpp
@@ -2330,24 +2330,24 @@ bool CWallet::TopUpKeyPool(unsigned int kpSize)
     return res;
 }
 
-BResult<CTxDestination> CWallet::GetNewDestination(const OutputType type, const std::string label)
+util::Result<CTxDestination> CWallet::GetNewDestination(const OutputType type, const std::string label)
 {
     LOCK(cs_wallet);
     auto spk_man = GetScriptPubKeyMan(type, false /* internal */);
     if (!spk_man) {
-        return strprintf(_("Error: No %s addresses available."), FormatOutputType(type));
+        return util::Error{strprintf(_("Error: No %s addresses available."), FormatOutputType(type))};
     }
 
     spk_man->TopUp();
     auto op_dest = spk_man->GetNewDestination(type);
     if (op_dest) {
-        SetAddressBook(op_dest.GetObj(), label, "receive");
+        SetAddressBook(*op_dest, label, "receive");
     }
 
     return op_dest;
 }
 
-BResult<CTxDestination> CWallet::GetNewChangeDestination(const OutputType type)
+util::Result<CTxDestination> CWallet::GetNewChangeDestination(const OutputType type)
 {
     LOCK(cs_wallet);
 
@@ -2355,7 +2355,7 @@ BResult<CTxDestination> CWallet::GetNewChangeDestination(const OutputType type)
     bilingual_str error;
     ReserveDestination reservedest(this, type);
     if (!reservedest.GetReservedDestination(dest, true, error)) {
-        return error;
+        return util::Error{error};
     }
 
     reservedest.KeepDestination();