private networks and utility functions

[vyzo]

extracted from go-libp2p-autonat

[raulk]

Heads-up, I'm working on the Routability() logic that we discussed on libp2p/go-libp2p#436 (comment), which overlaps with this PR.

Probably worth waiting until I'm done with that, because I'm also taking into consideration reserved and special purpose addresses. And I think the Routability API is more extensible going forward.

[Stebalien]

@raulk how long will that take? Basically, I asked @vyzo to do this because he would have otherwise exposed these as public methods from the autonat service (which I didn't want on the API). We could just do that and say "these aren't really public" but I'd prefer to do this the right way first.

[raulk]

Here's where I'm headed: raulk@5630917

I need to finishing populating the segment tables (using rfc6890 as a reference) and write tests.

Feel free to proceed with this PR if it's urgent. I'd be done today with Routability() anyway.

[vyzo]

I think we can extend IsPublicAddr to take into account all non-routable addrs.
IsPrivateAddr is fine only checking the private networks.

[vyzo]

Added all the unroutable address ranges in the IsPublicAddr test

[raulk]

Multicast ranges added in the SpecialPurpose segment: https://github.com/raulk/go-multiaddr-net/blob/routability/routability.go

Next step is to treat IPv4-IPv6 translation ranges properly.

[Stebalien]

(with my patches approved by @vyzo)