Break examples out into a new repo

[Stebalien]

This should reduce the code size of this repo and reduce the number of dependencies.

[Kubuxu]

There were discussions previously about what form this repo should take. For example, should it depend on everything and provide nice interface or should it just contain constructors and helper methods for using libp2p and so on. I can't recall conclusions of them.

@lgierth what do you think?

[Stebalien]

So, one motivation is that the DHT currently depends on this repo so we can't add any examples that use the DHT. (see #278). We could, alternatively:

• Put that example in the DHT repo.
• Move the parts of go-libp2p that the DHT actually needs (for tests) in a separate repo.
• Figure out a better way to deal with test dependencies (multiple gx packages in a single repo).

[anacrolix]

Looks like this was done in #381.