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This adds a `Component` helper type and a `ForEach` helper method.

The first attempt used an interface but interfaces imply allocation. We really
can't afford to allocate here.

This is a huge performance hit. Really, we just need to tell users not to modify
the result.

Also, get rid of an unnecessary pointer indirection (no api change).

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