Add new Creature::IsUsable function, which replaces IsFainted functionality in several places.

Signed-off-by: Deukhoofd <Deukhoofd@gmail.com>

src/Battling/Library/ExperienceLibrary.cpp

@@ -4,7 +4,7 @@
 void CreatureLib::Battling::ExperienceLibrary::HandleExperienceGain(
     CreatureLib::Battling::Creature* faintedMon,
     const std::unordered_set<ArbUt::BorrowedPtr<Creature>>& opponents) const {
-    for (auto opponent : opponents) {
+    for (const auto& opponent : opponents) {
         if (opponent->IsFainted())
             continue;
         if (!opponent->AllowedExperienceGain())