use utils.StringSet in more places

irc/accounts.go

 		return errAccountCreation
 	}
 
-	if restrictedCasefoldedNicks[casefoldedAccount] || restrictedSkeletons[skeleton] {
+	if restrictedCasefoldedNicks.Has(casefoldedAccount) || restrictedSkeletons.Has(skeleton) {
 		return errAccountAlreadyRegistered
 	}
 

irc/client_lookup_set.go

 			return "", errNicknameInvalid, false
 		}
 
-		if restrictedCasefoldedNicks[newCfNick] || restrictedSkeletons[newSkeleton] {
+		if restrictedCasefoldedNicks.Has(newCfNick) || restrictedSkeletons.Has(newSkeleton) {
 			return "", errNicknameInvalid, false
 		}
 

irc/nickname.go

 		"MemoServ", "BotServ", "OperServ",
 	}
 
-	restrictedCasefoldedNicks = make(map[string]bool)
-	restrictedSkeletons       = make(map[string]bool)
+	restrictedCasefoldedNicks = make(utils.StringSet)
+	restrictedSkeletons       = make(utils.StringSet)
 )
 
 func performNickChange(server *Server, client *Client, target *Client, session *Session, nickname string, rb *ResponseBuffer) error {

irc/services.go

 		if err != nil {
 			panic(err)
 		}
-		restrictedCasefoldedNicks[cfName] = true
+		restrictedCasefoldedNicks.Add(cfName)
 		skeleton, err := Skeleton(restrictedNickname)
 		if err != nil {
 			panic(err)
 		}
-		restrictedSkeletons[skeleton] = true
+		restrictedSkeletons.Add(skeleton)
 	}
 }