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/rpc/spend.cpp

@@ -158,14 +158,14 @@ UniValue SendMoney(CWallet& wallet, const CCoinControl &coin_control, std::vecto
     constexpr int RANDOM_CHANGE_POSITION = -1;
     auto res = CreateTransaction(wallet, recipients, RANDOM_CHANGE_POSITION, coin_control, true);
     if (!res) {
-        throw JSONRPCError(RPC_WALLET_INSUFFICIENT_FUNDS, res.GetError().original);
+        throw JSONRPCError(RPC_WALLET_INSUFFICIENT_FUNDS, util::ErrorString(res).original);
     }
-    const CTransactionRef& tx = res.GetObj().tx;
+    const CTransactionRef& tx = res->tx;
     wallet.CommitTransaction(tx, std::move(map_value), {} /* orderForm */);
     if (verbose) {
         UniValue entry(UniValue::VOBJ);
         entry.pushKV("txid", tx->GetHash().GetHex());
-        entry.pushKV("fee_reason", StringForFeeReason(res.GetObj().fee_calc.reason));
+        entry.pushKV("fee_reason", StringForFeeReason(res->fee_calc.reason));
         return entry;
     }
     return tx->GetHash().GetHex();