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This sucks but I can't think of a better way to do this. We really do want to
expose these features and doing so through type assertions is very go-like.

This method made implementing the manet.Listener interface annoying. Also, this
lets us drop the "use with caution" warning as we're now *wrapping* the listener
instead of exposing internal state.

Signed-off-by: Tom Swindell <t.swindell@rubyx.co.uk>

License: MIT
Signed-off-by: Hector Sanjuan <code@hector.link>

License: MIT
Signed-off-by: Juan Batiz-Benet <juan@benet.ai>

Implements:
- Conn
- Dial, Dialer
- Listen, Listener

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