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, 10 insertions, 10 deletions

diff --git a/src/wallet/scriptpubkeyman.cpp b/src/wallet/scriptpubkeyman.cpp
index bba31dfe0b..2f242901ab 100644
--- a/src/wallet/scriptpubkeyman.cpp
+++ b/src/wallet/scriptpubkeyman.cpp
@@ -21,10 +21,10 @@ namespace wallet {
 //! Value for the first BIP 32 hardened derivation. Can be used as a bit mask and as a value. See BIP 32 for more details.
 const uint32_t BIP32_HARDENED_KEY_LIMIT = 0x80000000;
 
-BResult<CTxDestination> LegacyScriptPubKeyMan::GetNewDestination(const OutputType type)
+util::Result<CTxDestination> LegacyScriptPubKeyMan::GetNewDestination(const OutputType type)
 {
     if (LEGACY_OUTPUT_TYPES.count(type) == 0) {
-        return _("Error: Legacy wallets only support the \"legacy\", \"p2sh-segwit\", and \"bech32\" address types");;
+        return util::Error{_("Error: Legacy wallets only support the \"legacy\", \"p2sh-segwit\", and \"bech32\" address types")};
     }
     assert(type != OutputType::BECH32M);
 
@@ -33,7 +33,7 @@ BResult<CTxDestination> LegacyScriptPubKeyMan::GetNewDestination(const OutputTyp
     // Generate a new key that is added to wallet
     CPubKey new_key;
     if (!GetKeyFromPool(new_key, type)) {
-        return _("Error: Keypool ran out, please call keypoolrefill first");
+        return util::Error{_("Error: Keypool ran out, please call keypoolrefill first")};
     }
     LearnRelatedScripts(new_key, type);
     return GetDestinationForKey(new_key, type);
@@ -1654,11 +1654,11 @@ std::set<CKeyID> LegacyScriptPubKeyMan::GetKeys() const
     return set_address;
 }
 
-BResult<CTxDestination> DescriptorScriptPubKeyMan::GetNewDestination(const OutputType type)
+util::Result<CTxDestination> DescriptorScriptPubKeyMan::GetNewDestination(const OutputType type)
 {
     // Returns true if this descriptor supports getting new addresses. Conditions where we may be unable to fetch them (e.g. locked) are caught later
     if (!CanGetAddresses()) {
-        return _("No addresses available");
+        return util::Error{_("No addresses available")};
     }
     {
         LOCK(cs_desc_man);
@@ -1676,11 +1676,11 @@ BResult<CTxDestination> DescriptorScriptPubKeyMan::GetNewDestination(const Outpu
         std::vector<CScript> scripts_temp;
         if (m_wallet_descriptor.range_end <= m_max_cached_index && !TopUp(1)) {
             // We can't generate anymore keys
-            return _("Error: Keypool ran out, please call keypoolrefill first");
+            return util::Error{_("Error: Keypool ran out, please call keypoolrefill first")};
         }
         if (!m_wallet_descriptor.descriptor->ExpandFromCache(m_wallet_descriptor.next_index, m_wallet_descriptor.cache, scripts_temp, out_keys)) {
             // We can't generate anymore keys
-            return _("Error: Keypool ran out, please call keypoolrefill first");
+            return util::Error{_("Error: Keypool ran out, please call keypoolrefill first")};
         }
 
         CTxDestination dest;
@@ -1766,11 +1766,11 @@ bool DescriptorScriptPubKeyMan::GetReservedDestination(const OutputType type, bo
     auto op_dest = GetNewDestination(type);
     index = m_wallet_descriptor.next_index - 1;
     if (op_dest) {
-        address = op_dest.GetObj();
+        address = *op_dest;
     } else {
-        error = op_dest.GetError();
+        error = util::ErrorString(op_dest);
     }
-    return op_dest.HasRes();
+    return bool(op_dest);
 }
 
 void DescriptorScriptPubKeyMan::ReturnDestination(int64_t index, bool internal, const CTxDestination& addr)