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

Add "ip6%" multiaddr support (IPv6 with zone ID)

- 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.

private networks and utility functions

(would have failed with ValueForProtocol)

This now means that addresses must *start* with the an IP address, we won't just
pull one out of the middle.

use multiaddr parsers

This allows us to do less string-parsing.

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.

fix loopback addresses

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

fix converting net.IPAddr to a multiaddr

Unfortunately, the existing abstraction couldn't handle the fact that the "ip"
net could parse to *either* an "/ip4/" or "/ip6/" multiaddr.

expose methods from underlying connection types

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.

turn NetListener() method on listener interface into a helper method

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.

Make the IP loopback check faster

Helps readers understand what they're looking at.

Decapsulate currently allocates a lot and this appears to have been causing some
issues (hard to tell, pprof is misbehaving).

Note: I removed the TODO as this function now *only* checks if we're dealing
with a loopback address. Not sure what `OverIPLoopback` was supposed to mean.

Note 2: Decapsulate actually checked if the IP6 loopback addresses
appeared *anywhere* in the multiadder. However, as we weren't doing this for
IP4, I decided to simplify this and only check prefixes.

use standard travis-ci scripts and enable sudo

This should fix CI.

Supersedes #34.