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It turns out we *were* relying on this. Furthermore, this is generally more
useful (especially given ip6zones).

- Add `FromIPAndZone` function (complementing the `FromIP` function).
- Handle zones when parsing `net.Addr`s.
- In `DialArgs`, return an error if an `ip6zone` is prefixed to an `ip4`
  or to another `ip6zone`.

Note: I was not able to add a listen test (in TestListenAddrs) for
`/zone/.../ip6/...` since there is no link-local address which is
guaranteed to exist on all systems.

1. Ensure we check the entire addr, not just a prefix.
2. Make the LinkLocal test skip zones.
3. Consider link-local multicast to be link-local.
4. Defer to *go* to determine if something is, in fact, a loopback/link-local address.

* fe80::1 is not a loopback address
* ::ffff:127.* probably is (IPv4-mapped IPv6)

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.

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

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

The error in String should not actually ocurr, as the multiaddr
should have been valid to be constructed successfully, and thus
should be encoded back to its string rep correctly.

This will be bolstered by creating an interface (to prevent messing
with the internal bytes)