This was showing up on CPU profiles as a significant source of repeated
allocations. We don't really care about over allocation (don't keep these slices
around) and should rarely have more than 8 protocols in a single address.
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
