pubsub.Topic should satisfy fmt.Stringer

[lthibault]

Due to the requirement of maintaining a single *pubsub.Topic handle per pubsub topic, it is often helpful to identify the topic to which a given handle belongs.

Common solutions include:

• map[string]*topic.Topic
• Defining a local topic handle that embeds *topic.Topic, and exposes the topic string via a String() string method

Seeing as how *topic.Topic already contains an unexported topic field, it would be both trivial and extremely helpful to add the following method:

func (t *Topic) String() string {
    return t.topic
}

I'm happy to submit a PR if there's any interest.

[aschmahmann]

Sounds great, was one of the suggestions here (https://github.com/libp2p/go-libp2p-pubsub/issues/198#issuecomment-548896902) that has yet to be implemented.

Would welcome a PR.

[lthibault]

@aschmahmann Done! Please see #370.