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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