GitLab

This commit changes the struct to a new Multiaddr interface:

```Go
type Multiaddr interface {
  Equal(Multiaddr) bool
  Bytes() []byte
  String() string
  Protocols() []*Protocol
  Encapsulate(Multiaddr) Multiaddr
  Decapsulate(Multiaddr) Multiaddr
}
```

This means a few things have changed:

- use Multiaddr interface, struct not exported
- Bytes returns a copy of the internal bytes
- Some methods no longer return errors (catch errors in NewMultiaddr)
  - String (panics if malformed)
  - Protocols (panics if malformed)
  - Decapsulate (no-op if not prefix)
- Moved net-specific functions to package
  - Multiaddr.DialArgs() -> DialArgs(Multiaddr)
  - Multiaddr.IsThinWaist() -> IsThinWaist(Multiaddr)

cc @whyrusleeping @perfmode