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/wallet/feebumper.cpp

@@ -221,11 +221,11 @@ Result CreateRateBumpTransaction(CWallet& wallet, const uint256& txid, const CCo
     constexpr int RANDOM_CHANGE_POSITION = -1;
     auto res = CreateTransaction(wallet, recipients, RANDOM_CHANGE_POSITION, new_coin_control, false);
     if (!res) {
-        errors.push_back(Untranslated("Unable to create transaction.") + Untranslated(" ") + res.GetError());
+        errors.push_back(Untranslated("Unable to create transaction.") + Untranslated(" ") + util::ErrorString(res));
         return Result::WALLET_ERROR;
     }
 
-    const auto& txr = res.GetObj();
+    const auto& txr = *res;
     // Write back new fee if successful
     new_fee = txr.fee;