Add toString override

mirai-core/src/commonMain/kotlin/net.mamoe.mirai/message/data/Message.kt

@@ -80,6 +80,8 @@ interface Message {
         else MessageChainImpl(this, tail)
     }
 
+    override fun toString(): String
+
     operator fun plus(another: Message): MessageChain = this.followedBy(another)
     operator fun plus(another: String): MessageChain = this.followedBy(another.toString().toMessage())
     // `+ ""` will be resolved to `plus(String)` instead of `plus(CharSeq)`

mirai-core/src/commonMain/kotlin/net.mamoe.mirai/message/data/MessageChain.kt

@@ -32,6 +32,8 @@ interface MessageChain : Message, MutableList<Message> {
         this.plusAssign(plain.toMessage())
     }
 
+    override fun toString(): String
+
     /**
      * 获取第一个类型为 [key] 的 [Message] 实例
      *